/* ============================================================
 * BabiaTech — 41_rust style ("The Rust Programming Language" book)
 *   — classic mdBook layout: thick left chapter tree sidebar,
 *     centered article column, system-stack body, heavier serif
 *     display headings, brand red replacing classic orange.
 * Brand red:  #EF4444   (used in place of mdBook orange #E37120)
 * Brand text: #1F1B17   (warm near-black, like rust book ink)
 * Paper bg:   #F8F4EB   (warm cream — distinct from pulse/peak)
 * Code bg:    #F1ECE0   (slightly darker cream)
 * ============================================================ */

/* ---------- 0. CSS custom props ---------- */
:root {
  --bt-brand:        #EF4444;
  --bt-brand-strong: #C62B2B;
  --bt-brand-soft:   #FBE0E0;
  --bt-brand-tint:   #FCE9E9;

  --bt-ink:          #1F1B17;
  --bt-ink-muted:    #5C5650;
  --bt-ink-soft:     #8A847B;
  --bt-line:         #E5DECF;
  --bt-line-strong:  #D2C8B2;

  --bt-paper:        #F8F4EB;
  --bt-paper-card:   #FFFFFF;
  --bt-paper-warm:   #F1ECE0;
  --bt-paper-deep:   #E8E0CC;

  --bt-link:         #C62B2B;
  --bt-link-hover:   #8A1F1F;
  --bt-visited:      #883333;

  --bt-code-bg:      #F1ECE0;
  --bt-code-ink:     #2A2620;
  --bt-code-border:  #DDD4BB;

  --bt-callout-note-bg:  #F4EDD8;
  --bt-callout-note-bd:  #DABE6F;
  --bt-callout-warn-bg:  #FCE9E9;
  --bt-callout-warn-bd:  #EF4444;

  --bt-radius-sm: 3px;
  --bt-radius:    5px;
  --bt-radius-lg: 8px;

  --bt-sidebar-w: 308px;
  --bt-canvas-pad: 4rem;
  --bt-content-w: 720px;
  --bt-toc-w: 220px;

  --bt-shadow-sm: 0 1px 0 rgba(31,27,23,0.05);
  --bt-shadow-md: 0 2px 8px rgba(31,27,23,0.08);
  --bt-shadow-lg: 0 4px 24px rgba(31,27,23,0.10);

  --bt-font-serif: "Source Serif Pro", "Source Serif", "Iowan Old Style", "Apple Garamond", "Baskerville", "Times New Roman", "Droid Serif", Times, "Source Han Serif SC", "Noto Serif SC", serif;
  --bt-font-sans: "Open Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "PingFang SC", "Microsoft YaHei", sans-serif;
  --bt-font-mono: "Source Code Pro", Consolas, "Ubuntu Mono", Menlo, "DejaVu Sans Mono", monospace, "Apple Color Emoji";

  --bt-z-header: 50;
  --bt-z-sidebar: 40;
  --bt-z-mobile-overlay: 60;
  --bt-z-langmenu: 70;
}

[data-theme="rust"] {
  --bt-paper:      #FCFAF6;
  --bt-paper-card: #FFFFFF;
  --bt-paper-warm: #F7F1E5;
  --bt-ink:        #262019;
  --bt-line:       #E0D6C0;
}

[data-theme="dark"] {
  --bt-ink:          #E8E2D5;
  --bt-ink-muted:    #B8B0A0;
  --bt-ink-soft:     #847C70;
  --bt-line:         #2A2520;
  --bt-line-strong:  #3A342C;

  --bt-paper:        #1A1612;
  --bt-paper-card:   #221E18;
  --bt-paper-warm:   #2A2520;
  --bt-paper-deep:   #312B23;

  --bt-link:         #FF7A7A;
  --bt-link-hover:   #FF9A9A;
  --bt-visited:      #D88080;

  --bt-code-bg:      #2A2520;
  --bt-code-ink:     #F0EADC;
  --bt-code-border:  #3A342C;

  --bt-callout-note-bg: #2D2A1F;
  --bt-callout-note-bd: #B89A50;
  --bt-callout-warn-bg: #3A2020;
  --bt-callout-warn-bd: #EF4444;

  --bt-shadow-sm: 0 1px 0 rgba(0,0,0,0.4);
  --bt-shadow-md: 0 2px 12px rgba(0,0,0,0.5);
  --bt-shadow-lg: 0 4px 32px rgba(0,0,0,0.6);
}

/* ---------- 1. Reset & base ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; text-size-adjust: 100%; }
body {
  margin: 0;
  font-family: var(--bt-font-sans);
  font-size: 16px;
  line-height: 1.7;
  color: var(--bt-ink);
  background: var(--bt-paper);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}
img, svg { max-width: 100%; height: auto; vertical-align: middle; }
a { color: var(--bt-link); text-decoration: none; transition: color .12s ease; }
a:hover { color: var(--bt-link-hover); text-decoration: underline; text-underline-offset: 2px; }
a:focus-visible { outline: 2px solid var(--bt-brand); outline-offset: 2px; border-radius: 2px; }
button { font: inherit; color: inherit; cursor: pointer; background: transparent; border: 0; padding: 0; }
hr { border: 0; border-top: 1px solid var(--bt-line); margin: 2.4rem 0; }
::selection { background: var(--bt-brand-soft); color: var(--bt-ink); }

/* ---------- 2. Skip link ---------- */
.bt-skip {
  position: absolute;
  top: -100px; left: 1rem;
  padding: .6rem 1rem;
  background: var(--bt-ink);
  color: var(--bt-paper);
  border-radius: var(--bt-radius);
  z-index: 999;
}
.bt-skip:focus { top: 0.6rem; color: var(--bt-paper); }

/* ---------- 3. Layout shell (mdBook style) ---------- */
.bt-body { min-height: 100vh; display: flex; flex-direction: column; }
.bt-shell { display: flex; flex: 1 0 auto; min-height: 0; }
.bt-canvas {
  flex: 1 1 auto;
  min-width: 0;
  display: flex;
  flex-direction: column;
  background: var(--bt-paper);
}
.bt-main {
  flex: 1 0 auto;
  width: 100%;
  max-width: calc(var(--bt-content-w) + var(--bt-toc-w) + 4rem);
  margin: 0 auto;
  padding: 1.4rem var(--bt-canvas-pad) 4rem;
}

/* ---------- 4. Header (top toolbar) ---------- */
.bt-header {
  position: sticky;
  top: 0;
  z-index: var(--bt-z-header);
  background: var(--bt-paper);
  border-bottom: 1px solid var(--bt-line);
  height: 50px;
}
.bt-header__bar {
  height: 50px;
  display: grid;
  grid-template-columns: auto 1fr auto;
  align-items: center;
  padding: 0 1.2rem;
  gap: 0.5rem;
}
.bt-header__left, .bt-header__right {
  display: flex; align-items: center; gap: 0.25rem;
}
.bt-header__center { text-align: center; min-width: 0; }
.bt-icon-btn {
  width: 38px; height: 38px;
  display: inline-flex; align-items: center; justify-content: center;
  border-radius: var(--bt-radius);
  color: var(--bt-ink-muted);
  transition: background .15s, color .15s;
}
.bt-icon-btn:hover {
  background: var(--bt-paper-warm);
  color: var(--bt-ink);
}
.bt-icon-btn--print { text-decoration: none; }
.bt-brand {
  font-family: var(--bt-font-serif);
  color: var(--bt-ink-muted);
  text-decoration: none;
  font-size: 1.05rem;
  font-weight: 400;
  letter-spacing: 0.02em;
}
.bt-brand:hover { color: var(--bt-ink); text-decoration: none; }
.bt-brand__title { font-style: italic; }

/* Language menu */
.bt-lang { position: relative; }
.bt-lang__btn {
  display: inline-flex; align-items: center; gap: 6px;
  height: 38px; padding: 0 12px;
  border-radius: var(--bt-radius);
  color: var(--bt-ink-muted);
  font-size: 0.85rem;
}
.bt-lang__btn:hover { background: var(--bt-paper-warm); color: var(--bt-ink); }
.bt-lang__cur { font-weight: 500; }
.bt-lang__menu {
  position: absolute;
  top: calc(100% + 4px); right: 0;
  min-width: 170px;
  margin: 0; padding: 6px;
  list-style: none;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius);
  box-shadow: var(--bt-shadow-md);
  z-index: var(--bt-z-langmenu);
  display: none;
}
.bt-lang:hover .bt-lang__menu,
.bt-lang:focus-within .bt-lang__menu { display: block; }
.bt-lang__menu li { padding: 0; }
.bt-lang__menu a {
  display: block;
  padding: 6px 10px;
  border-radius: 3px;
  font-size: 0.85rem;
  color: var(--bt-ink);
  text-decoration: none;
}
.bt-lang__menu a:hover { background: var(--bt-paper-warm); color: var(--bt-link); }
.bt-lang__menu a.is-current {
  background: var(--bt-brand-soft);
  color: var(--bt-brand-strong);
  font-weight: 500;
}

/* ---------- 5. Sidebar (mdBook chapter tree) ---------- */
.bt-sidebar {
  width: var(--bt-sidebar-w);
  flex-shrink: 0;
  background: var(--bt-paper);
  border-right: 1px solid var(--bt-line);
  height: calc(100vh - 50px);
  position: sticky;
  top: 50px;
  overflow-y: auto;
  font-size: 14px;
  line-height: 1.5;
}
.bt-sidebar__brand {
  padding: 1.4rem 1.4rem 0.8rem;
  border-bottom: 1px solid var(--bt-line);
}
.bt-sidebar__logo {
  display: inline-flex; align-items: center; gap: 8px;
  text-decoration: none;
  color: var(--bt-ink);
}
.bt-sidebar__logo:hover { text-decoration: none; color: var(--bt-brand); }
.bt-sidebar__logo-icon { display: inline-flex; }
.bt-sidebar__logo-text {
  font-family: var(--bt-font-serif);
  font-size: 1.05rem;
  font-style: italic;
  letter-spacing: 0.01em;
}
.bt-sidebar__filter {
  position: relative;
  padding: 0.8rem 1rem;
  border-bottom: 1px solid var(--bt-line);
}
.bt-sidebar__filter-icon {
  position: absolute;
  left: 1.6rem; top: 50%;
  transform: translateY(-50%);
  color: var(--bt-ink-soft);
  pointer-events: none;
}
.bt-sidebar__filter-input {
  width: 100%;
  padding: 6px 10px 6px 32px;
  border: 1px solid var(--bt-line);
  background: var(--bt-paper-warm);
  border-radius: var(--bt-radius);
  font-size: 13px;
  color: var(--bt-ink);
}
.bt-sidebar__filter-input:focus {
  outline: 0;
  border-color: var(--bt-brand);
  background: var(--bt-paper-card);
}
.bt-sidebar__inner {
  padding: 0.8rem 0 2.5rem;
}
.bt-sidebar__list {
  list-style: none;
  margin: 0;
  padding: 0;
}
.bt-sidebar__list--top { padding: 0.4rem 0.6rem 0.6rem; }
.bt-sidebar__list--inner {
  padding: 0.2rem 0 0.4rem 1.6rem;
  margin-bottom: 0.4rem;
}
.bt-sidebar__link {
  display: block;
  padding: 5px 10px 5px 14px;
  color: var(--bt-link);
  text-decoration: none;
  border-radius: 0;
  border-left: 3px solid transparent;
  transition: background .12s, border-color .12s;
}
.bt-sidebar__link:hover {
  background: var(--bt-paper-warm);
  text-decoration: none;
}
.bt-sidebar__link.is-active {
  color: var(--bt-link);
  background: var(--bt-brand-tint);
  border-left-color: var(--bt-brand);
  font-weight: 500;
}
.bt-sidebar__link--top {
  font-weight: 500;
  color: var(--bt-link);
  padding-left: 12px;
}
.bt-sidebar__group {
  border-top: 1px solid var(--bt-line);
  padding: 0.45rem 0;
}
.bt-sidebar__heading {
  display: flex;
  align-items: baseline;
  gap: 8px;
  padding: 6px 1rem 6px 14px;
  font-weight: 700;
  font-size: 14px;
  color: var(--bt-ink);
  text-decoration: none;
  cursor: pointer;
  border-left: 3px solid transparent;
}
.bt-sidebar__heading:hover {
  background: var(--bt-paper-warm);
  text-decoration: none;
  color: var(--bt-ink);
}
.bt-sidebar__heading.is-active {
  background: var(--bt-brand-tint);
  border-left-color: var(--bt-brand);
  color: var(--bt-brand-strong);
}
.bt-sidebar__num {
  font-weight: 700;
  color: var(--bt-ink);
  font-size: 14px;
  min-width: 22px;
}
.bt-sidebar__cat { flex: 1 1 auto; }
.bt-sidebar__sub-num {
  display: inline-block;
  min-width: 30px;
  font-weight: 600;
  font-size: 13px;
  color: var(--bt-ink-muted);
}
.bt-sidebar__sub-text {
  display: inline;
}
.bt-sidebar__link--sub {
  display: block;
  padding: 4px 10px 4px 0;
  font-size: 13.5px;
  line-height: 1.5;
  color: var(--bt-link);
  border-left: none;
  border-radius: 0;
  word-break: break-word;
}
.bt-sidebar__link--sub.is-active {
  color: var(--bt-brand-strong);
  font-weight: 600;
  background: transparent;
  border-left: none;
}
.bt-sidebar__link--sub.is-active .bt-sidebar__sub-num { color: var(--bt-brand-strong); }
.bt-sidebar__group.is-collapsed .bt-sidebar__list--inner { display: none; }
.bt-sidebar__group.is-collapsed .bt-sidebar__heading::before { content: "▸ "; color: var(--bt-ink-soft); margin-right: -2px; font-size: 10px; }

/* ---------- 6. Article container ---------- */
.bt-article {
  max-width: var(--bt-content-w);
  margin: 0 auto;
  padding: 1rem 0;
}
.bt-article--list, .bt-article--cat, .bt-article--page, .bt-article--post {
  max-width: var(--bt-content-w);
}
.bt-article--home { max-width: 760px; }

/* Breadcrumb */
.bt-crumb {
  font-size: 13.5px;
  color: var(--bt-ink-muted);
  margin: 0 0 1.2rem;
  padding-bottom: 0.6rem;
  border-bottom: 1px dashed var(--bt-line);
}
.bt-crumb a { color: var(--bt-ink-muted); }
.bt-crumb a:hover { color: var(--bt-link); }
.bt-crumb__sep { margin: 0 6px; color: var(--bt-ink-soft); }
.bt-crumb__current { color: var(--bt-ink); }

/* Article head — heavy serif, classic mdBook */
.bt-article__head {
  margin: 0 0 1.6rem;
}
.bt-article__chap {
  font-family: var(--bt-font-serif);
  font-size: 1.05rem;
  font-weight: 400;
  font-style: italic;
  color: var(--bt-ink-muted);
  margin-bottom: 0.4rem;
  letter-spacing: 0.04em;
}
.bt-article__h1 {
  font-family: var(--bt-font-serif);
  font-size: 2.4rem;
  font-weight: 700;
  line-height: 1.2;
  letter-spacing: -0.005em;
  color: var(--bt-ink);
  margin: 0 0 0.4rem;
}
.bt-article__lede {
  font-family: var(--bt-font-serif);
  font-style: italic;
  font-size: 1.06rem;
  line-height: 1.6;
  color: var(--bt-ink-muted);
  margin: 0.4rem 0 0.8rem;
}
.bt-article__meta {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 0.4rem;
  font-size: 13px;
  color: var(--bt-ink-muted);
  padding: 0.4rem 0 0.2rem;
  border-top: 1px dashed var(--bt-line);
}
.bt-article__meta-item time { color: var(--bt-ink); }
.bt-article__meta-sep { color: var(--bt-ink-soft); }
.bt-chip {
  display: inline-block;
  padding: 1px 8px;
  background: var(--bt-brand-tint);
  color: var(--bt-brand-strong);
  border-radius: 12px;
  font-size: 12px;
  font-weight: 500;
  text-decoration: none;
}
.bt-chip:hover { background: var(--bt-brand-soft); color: var(--bt-brand-strong); text-decoration: none; }

/* Cover (home) — Rust Book cover style */
.bt-article__head--cover { margin-bottom: 2rem; }
.bt-cover {
  background: linear-gradient(180deg, var(--bt-paper-warm) 0%, var(--bt-paper-card) 100%);
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius-lg);
  padding: 2.6rem 2rem 2.2rem;
  text-align: center;
  box-shadow: var(--bt-shadow-sm);
}
.bt-cover__edition {
  font-family: var(--bt-font-serif);
  font-style: italic;
  font-size: 13px;
  color: var(--bt-ink-soft);
  letter-spacing: 0.04em;
  margin-bottom: 0.8rem;
}
.bt-cover__title {
  font-family: var(--bt-font-serif);
  font-size: 2.5rem;
  font-weight: 700;
  line-height: 1.18;
  color: var(--bt-ink);
  margin: 0.2rem auto 0.7rem;
  max-width: 580px;
}
.bt-cover__author {
  font-family: var(--bt-font-serif);
  font-style: italic;
  font-size: 1rem;
  color: var(--bt-ink-muted);
  margin: 0 0 1rem;
}
.bt-cover__lede {
  font-size: 0.98rem;
  color: var(--bt-ink-muted);
  margin: 0 auto 1.4rem;
  max-width: 580px;
  line-height: 1.65;
}
.bt-cover__cta {
  display: inline-flex;
  gap: 0.7rem;
  flex-wrap: wrap;
  justify-content: center;
  margin-top: 0.4rem;
}

/* Hero search box (in cover) */
.bt-hero-search {
  display: flex;
  align-items: center;
  position: relative;
  margin: 0 auto 1.2rem;
  max-width: 460px;
}
.bt-hero-search__icon {
  position: absolute; left: 14px; top: 50%; transform: translateY(-50%);
  color: var(--bt-ink-soft);
  display: flex;
}
.bt-hero-search__input {
  width: 100%;
  padding: 11px 14px 11px 40px;
  font-size: 14.5px;
  border: 1px solid var(--bt-line-strong);
  background: var(--bt-paper-card);
  border-radius: 999px;
  color: var(--bt-ink);
}
.bt-hero-search__input:focus {
  outline: 0;
  border-color: var(--bt-brand);
  box-shadow: 0 0 0 3px var(--bt-brand-tint);
}

/* Buttons */
.bt-btn {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 9px 18px;
  border-radius: var(--bt-radius);
  font-weight: 500;
  font-size: 14px;
  text-decoration: none;
  border: 1px solid transparent;
  transition: background .15s, color .15s, border-color .15s;
  white-space: nowrap;
}
.bt-btn--primary {
  background: var(--bt-brand);
  color: #FFFFFF;
  border-color: var(--bt-brand);
}
.bt-btn--primary:hover {
  background: var(--bt-brand-strong);
  border-color: var(--bt-brand-strong);
  color: #FFFFFF;
  text-decoration: none;
}
.bt-btn--ghost {
  background: var(--bt-paper-card);
  color: var(--bt-ink);
  border-color: var(--bt-line-strong);
}
.bt-btn--ghost:hover {
  background: var(--bt-paper-warm);
  border-color: var(--bt-ink-muted);
  color: var(--bt-ink);
  text-decoration: none;
}

/* ---------- 7. Prose / typography ---------- */
.bt-prose {
  font-size: 16px;
  line-height: 1.8;
  color: var(--bt-ink);
}
.bt-prose > * + * { margin-top: 1rem; }
.bt-prose h2, .bt-prose h3, .bt-prose h4 {
  font-family: var(--bt-font-serif);
  color: var(--bt-ink);
  line-height: 1.32;
  scroll-margin-top: 70px;
}
.bt-prose h2 {
  font-size: 1.8rem;
  font-weight: 700;
  margin-top: 2.4rem;
  padding-bottom: 0.35rem;
  border-bottom: 1px solid var(--bt-line);
}
.bt-prose h3 {
  font-size: 1.35rem;
  font-weight: 700;
  margin-top: 1.9rem;
}
.bt-prose h4 {
  font-size: 1.08rem;
  font-weight: 700;
  margin-top: 1.5rem;
}
.bt-prose h2 a, .bt-prose h3 a, .bt-prose h4 a {
  color: inherit;
  text-decoration: none;
}
.bt-prose h2 a:hover, .bt-prose h3 a:hover, .bt-prose h4 a:hover {
  color: var(--bt-link);
  text-decoration: none;
}
.bt-prose p {
  margin: 0 0 1rem;
}
.bt-prose strong {
  font-weight: 700;
  color: var(--bt-ink);
}
.bt-prose em {
  font-style: italic;
  font-family: var(--bt-font-serif);
}
.bt-prose ul, .bt-prose ol {
  margin: 0 0 1rem;
  padding-left: 1.6rem;
}
.bt-prose li { margin: 0.25rem 0; }
.bt-prose li > ul, .bt-prose li > ol { margin: 0.2rem 0 0.4rem; }
.bt-prose blockquote {
  margin: 1.2rem 0;
  padding: 0.6rem 1.2rem;
  border-left: 4px solid var(--bt-brand);
  background: var(--bt-paper-warm);
  color: var(--bt-ink-muted);
  font-style: italic;
}
.bt-prose blockquote p:last-child { margin-bottom: 0; }
.bt-prose a { color: var(--bt-link); text-decoration: underline; text-decoration-color: rgba(198,43,43,0.4); text-underline-offset: 2px; }
.bt-prose a:hover { color: var(--bt-link-hover); text-decoration-color: var(--bt-link-hover); }

/* Inline code */
.bt-prose code {
  font-family: var(--bt-font-mono);
  font-size: 0.92em;
  padding: 0.12em 0.36em;
  background: var(--bt-code-bg);
  color: var(--bt-code-ink);
  border-radius: 3px;
  border: 1px solid var(--bt-code-border);
  word-break: break-word;
}

/* Code block — mdBook-ish */
.bt-prose pre {
  position: relative;
  margin: 1.4rem 0;
  padding: 1rem 1.1rem;
  background: var(--bt-code-bg);
  border: 1px solid var(--bt-code-border);
  border-radius: var(--bt-radius);
  overflow-x: auto;
  font-size: 0.92rem;
  line-height: 1.65;
}
.bt-prose pre code {
  padding: 0;
  background: transparent;
  border: 0;
  color: var(--bt-code-ink);
  font-size: inherit;
}
.bt-copy {
  position: absolute;
  top: 6px; right: 8px;
  padding: 3px 9px;
  font-size: 11px;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-code-border);
  border-radius: 3px;
  color: var(--bt-ink-muted);
  opacity: 0;
  transition: opacity .15s;
}
.bt-prose pre:hover .bt-copy,
.bt-prose pre .bt-copy:focus-visible { opacity: 1; }
.bt-copy:hover { color: var(--bt-ink); }

/* Tables */
.bt-prose table {
  width: 100%;
  margin: 1.4rem 0;
  border-collapse: collapse;
  font-size: 0.95rem;
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius);
  overflow: hidden;
}
.bt-prose thead {
  background: var(--bt-paper-warm);
  border-bottom: 2px solid var(--bt-line-strong);
}
.bt-prose th, .bt-prose td {
  padding: 9px 14px;
  text-align: left;
  border-bottom: 1px solid var(--bt-line);
  vertical-align: top;
}
.bt-prose th {
  font-weight: 700;
  color: var(--bt-ink);
  font-family: var(--bt-font-serif);
}
.bt-prose tbody tr:nth-child(even) { background: rgba(241,236,224,0.45); }
.bt-prose tbody tr:hover { background: var(--bt-brand-tint); }

/* Callouts */
.bt-callout {
  display: flex;
  gap: 0.75rem;
  margin: 1.4rem 0;
  padding: 0.85rem 1.1rem;
  border-left: 4px solid;
  border-radius: 0 var(--bt-radius) var(--bt-radius) 0;
  font-size: 0.95rem;
}
.bt-callout--note {
  background: var(--bt-callout-note-bg);
  border-color: var(--bt-callout-note-bd);
  color: var(--bt-ink);
}
.bt-callout--warn {
  background: var(--bt-callout-warn-bg);
  border-color: var(--bt-callout-warn-bd);
  color: var(--bt-ink);
}
.bt-callout strong { color: var(--bt-ink); }

/* ---------- 8. Article body w/ TOC ---------- */
.bt-article__body {
  display: grid;
  grid-template-columns: minmax(0, 1fr) var(--bt-toc-w);
  gap: 2.5rem;
  align-items: start;
}
.bt-article__body > .bt-prose { min-width: 0; }
.bt-toc {
  position: sticky;
  top: 70px;
  max-height: calc(100vh - 90px);
  overflow-y: auto;
  font-size: 13.5px;
  padding-left: 1.2rem;
  border-left: 1px solid var(--bt-line);
}
.bt-toc__inner { padding: 0; }
.bt-toc__head {
  font-family: var(--bt-font-serif);
  font-size: 0.85rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--bt-ink-soft);
  margin-bottom: 0.6rem;
}
.bt-toc__list {
  list-style: none;
  margin: 0 0 1.4rem;
  padding: 0;
}
.bt-toc__list--plain { margin: 0; }
.bt-toc__item {
  margin: 4px 0;
  line-height: 1.45;
}
.bt-toc__item--lv2 { padding-left: 0; }
.bt-toc__item--lv3 { padding-left: 14px; font-size: 12.5px; }
.bt-toc__item a {
  display: block;
  padding: 2px 0;
  color: var(--bt-ink-muted);
  text-decoration: none;
  border-left: 2px solid transparent;
  padding-left: 8px;
  margin-left: -8px;
}
.bt-toc__item a:hover {
  color: var(--bt-link);
  text-decoration: none;
  border-left-color: var(--bt-brand-soft);
}
.bt-toc__item a.is-active {
  color: var(--bt-brand-strong);
  border-left-color: var(--bt-brand);
  font-weight: 600;
}
.bt-toc__more { margin-top: 1.4rem; padding-top: 1rem; border-top: 1px solid var(--bt-line); }
.bt-toc__more .bt-toc__list--plain a { color: var(--bt-link); }

/* ---------- 9. Prev / Next (large arrows) ---------- */
.bt-prevnext {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
  margin: 3rem 0 1rem;
  padding-top: 1.4rem;
  border-top: 1px solid var(--bt-line);
}
.bt-prevnext__item {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 0.9rem 1rem;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius);
  text-decoration: none;
  color: var(--bt-ink);
  transition: background .15s, border-color .15s, color .15s;
}
.bt-prevnext__item:hover {
  background: var(--bt-brand-tint);
  border-color: var(--bt-brand-soft);
  color: var(--bt-brand-strong);
  text-decoration: none;
}
.bt-prevnext__next { flex-direction: row-reverse; text-align: right; }
.bt-prevnext__next .bt-prevnext__body { text-align: right; }
.bt-prevnext__arrow {
  flex-shrink: 0;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 30px; height: 30px;
  color: var(--bt-ink-muted);
}
.bt-prevnext__item:hover .bt-prevnext__arrow { color: var(--bt-brand); }
.bt-prevnext__body { min-width: 0; }
.bt-prevnext__lab {
  display: block;
  font-size: 11.5px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--bt-ink-soft);
  margin-bottom: 2px;
}
.bt-prevnext__t {
  display: block;
  font-family: var(--bt-font-serif);
  font-weight: 600;
  font-size: 14.5px;
  line-height: 1.35;
  color: inherit;
  overflow: hidden;
  text-overflow: ellipsis;
  display: -webkit-box;
  -webkit-line-clamp: 2;
  -webkit-box-orient: vertical;
}
.bt-prevnext__placeholder { display: block; }

/* ---------- 10. Home — chapter list ---------- */
.bt-chapters {
  list-style: none;
  margin: 1.2rem 0;
  padding: 0;
  counter-reset: bt-chap;
  display: grid;
  gap: 0.4rem;
}
.bt-chapters__item { padding: 0; }
.bt-chapters__link {
  display: flex;
  align-items: flex-start;
  gap: 14px;
  padding: 0.85rem 1rem 0.9rem;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius);
  text-decoration: none;
  color: var(--bt-ink);
  transition: background .15s, border-color .15s;
}
.bt-chapters__link:hover {
  background: var(--bt-paper-warm);
  border-color: var(--bt-line-strong);
  text-decoration: none;
  color: var(--bt-ink);
}
.bt-chapters__num {
  flex-shrink: 0;
  font-family: var(--bt-font-serif);
  font-size: 1.5rem;
  font-weight: 700;
  line-height: 1;
  color: var(--bt-brand);
  min-width: 36px;
  padding-top: 1px;
}
.bt-chapters__body {
  flex: 1 1 auto;
  min-width: 0;
}
.bt-chapters__title {
  display: block;
  font-family: var(--bt-font-serif);
  font-size: 1.15rem;
  font-weight: 700;
  color: var(--bt-ink);
  margin-bottom: 2px;
}
.bt-chapters__desc {
  display: block;
  font-size: 13.5px;
  color: var(--bt-ink-muted);
  line-height: 1.55;
}
.bt-chapters__sub {
  display: block;
  margin-top: 6px;
  font-size: 12.5px;
  color: var(--bt-ink-soft);
  line-height: 1.5;
}
.bt-chapters__sub-item {
  display: inline-block;
  margin-right: 12px;
}
.bt-chapters__count {
  display: inline-block;
  margin-top: 6px;
  padding: 1px 7px;
  background: var(--bt-brand-tint);
  color: var(--bt-brand-strong);
  border-radius: 10px;
  font-size: 11.5px;
  font-weight: 500;
}

/* Reflist (small linked notes) */
.bt-reflist {
  list-style: none;
  margin: 1rem 0;
  padding: 0.4rem 0;
  border-top: 1px solid var(--bt-line);
  border-bottom: 1px solid var(--bt-line);
}
.bt-reflist li {
  padding: 0.4rem 0;
  border-bottom: 1px dashed var(--bt-line);
  font-size: 0.95rem;
  line-height: 1.6;
}
.bt-reflist li:last-child { border-bottom: 0; }

/* Post list */
.bt-postlist {
  list-style: none;
  margin: 1rem 0;
  padding: 0;
}
.bt-postlist--numbered { counter-reset: bt-post; }
.bt-postlist__item {
  padding: 1rem 0;
  border-bottom: 1px solid var(--bt-line);
  position: relative;
}
.bt-postlist__item:last-child { border-bottom: 0; }
.bt-postlist__num {
  display: inline-block;
  font-family: var(--bt-font-serif);
  font-weight: 700;
  font-size: 14px;
  color: var(--bt-brand);
  margin-right: 10px;
  min-width: 38px;
}
.bt-postlist__title {
  display: inline-block;
  font-family: var(--bt-font-serif);
  font-weight: 700;
  font-size: 1.1rem;
  line-height: 1.4;
  color: var(--bt-ink);
  text-decoration: none;
}
.bt-postlist__title:hover {
  color: var(--bt-link);
  text-decoration: underline;
  text-underline-offset: 3px;
}
.bt-postlist__meta {
  margin-top: 4px;
  font-size: 12.5px;
  color: var(--bt-ink-soft);
  display: flex;
  align-items: center;
  gap: 10px;
}
.bt-postlist__date { font-variant-numeric: tabular-nums; }
.bt-postlist__chip {
  display: inline-block;
  padding: 1px 7px;
  background: var(--bt-paper-warm);
  color: var(--bt-ink-muted);
  border-radius: 10px;
  font-size: 11.5px;
}
.bt-postlist__desc {
  margin: 6px 0 0;
  font-size: 13.5px;
  color: var(--bt-ink-muted);
  line-height: 1.55;
}
.bt-viewall {
  margin: 1rem 0;
  text-align: right;
  font-size: 14px;
}

/* FAQ */
.bt-faq { margin: 1rem 0; }
.bt-faq__item {
  border-bottom: 1px solid var(--bt-line);
  padding: 0.4rem 0;
}
.bt-faq__item:first-child { border-top: 1px solid var(--bt-line); }
.bt-faq__q {
  display: flex;
  gap: 8px;
  padding: 0.6rem 0.4rem;
  cursor: pointer;
  font-family: var(--bt-font-serif);
  font-size: 1.02rem;
  font-weight: 700;
  color: var(--bt-ink);
  list-style: none;
  position: relative;
}
.bt-faq__q::-webkit-details-marker { display: none; }
.bt-faq__q::after {
  content: "+";
  position: absolute;
  right: 0.4rem; top: 0.45rem;
  font-size: 18px;
  color: var(--bt-brand);
  font-family: var(--bt-font-mono);
  transition: transform .2s;
}
.bt-faq__item[open] .bt-faq__q::after { content: "−"; }
.bt-faq__q-num {
  font-family: var(--bt-font-mono);
  font-size: 13px;
  color: var(--bt-brand);
  min-width: 32px;
}
.bt-faq__a {
  padding: 0 0.4rem 0.6rem 40px;
  font-size: 0.95rem;
  color: var(--bt-ink-muted);
  line-height: 1.7;
}
.bt-faq__a p { margin: 0 0 0.6rem; }
.bt-faq__a p:last-child { margin: 0; }

/* Pager */
.bt-pager {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  flex-wrap: wrap;
  margin: 2.4rem 0 1rem;
  padding: 1.2rem 0 0;
  border-top: 1px solid var(--bt-line);
}
.bt-pager__btn, .bt-pager__num {
  display: inline-flex;
  align-items: center;
  padding: 6px 12px;
  font-size: 13.5px;
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius);
  background: var(--bt-paper-card);
  color: var(--bt-ink);
  text-decoration: none;
  min-width: 36px;
  justify-content: center;
}
.bt-pager__btn:hover, .bt-pager__num:hover {
  background: var(--bt-brand-tint);
  border-color: var(--bt-brand-soft);
  color: var(--bt-brand-strong);
  text-decoration: none;
}
.bt-pager__num.is-current {
  background: var(--bt-brand);
  border-color: var(--bt-brand);
  color: #FFFFFF;
}
.bt-pager__btn.is-disabled,
.bt-pager__num.is-disabled {
  background: var(--bt-paper-warm);
  border-color: var(--bt-line);
  color: var(--bt-ink-soft);
  pointer-events: none;
}

/* ---------- 11. Footer ---------- */
.bt-footer {
  margin-top: auto;
  background: var(--bt-paper-warm);
  border-top: 1px solid var(--bt-line-strong);
  font-size: 14px;
  color: var(--bt-ink-muted);
}
.bt-footer__inner {
  max-width: 1180px;
  margin: 0 auto;
  padding: 2.2rem 2rem 1.4rem;
}
.bt-footer__cols {
  display: grid;
  grid-template-columns: 1.4fr repeat(4, 1fr);
  gap: 1.6rem;
}
.bt-footer__col--brand {
  font-size: 13.5px;
}
.bt-footer__logo {
  font-family: var(--bt-font-serif);
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--bt-ink);
  margin-bottom: 6px;
}
.bt-footer__tag {
  margin: 0 0 0.4rem;
  line-height: 1.6;
}
.bt-footer__hint {
  margin: 0;
  font-style: italic;
  font-family: var(--bt-font-serif);
  color: var(--bt-ink-soft);
}
.bt-footer__h {
  font-family: var(--bt-font-serif);
  font-weight: 700;
  font-size: 14.5px;
  color: var(--bt-ink);
  margin: 0 0 8px;
  text-transform: capitalize;
}
.bt-footer__list, .bt-footer__lang {
  list-style: none;
  margin: 0;
  padding: 0;
}
.bt-footer__list li, .bt-footer__lang li { padding: 3px 0; }
.bt-footer__list a, .bt-footer__lang a {
  color: var(--bt-ink-muted);
  font-size: 13.5px;
  text-decoration: none;
}
.bt-footer__list a:hover, .bt-footer__lang a:hover {
  color: var(--bt-link);
  text-decoration: underline;
}
.bt-footer__lang a.is-current {
  color: var(--bt-brand-strong);
  font-weight: 600;
}
.bt-footer__bottom {
  margin-top: 1.6rem;
  padding-top: 1rem;
  border-top: 1px solid var(--bt-line);
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  gap: 0.8rem;
  font-size: 12.5px;
  color: var(--bt-ink-soft);
}
.bt-footer__rights, .bt-footer__risk {
  flex: 1 1 auto;
  min-width: 220px;
  line-height: 1.6;
}
.bt-footer__risk { text-align: right; max-width: 60%; }

/* ---------- 12. 404 ---------- */
.bt-404 {
  max-width: 480px;
  margin: 4rem auto;
  padding: 2.8rem 2rem;
  text-align: center;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius-lg);
}
.bt-404__code {
  font-family: var(--bt-font-serif);
  font-size: 4.5rem;
  font-weight: 700;
  color: var(--bt-brand);
  line-height: 1;
}
.bt-404__title {
  font-family: var(--bt-font-serif);
  font-size: 1.5rem;
  margin: 0.4rem 0 0.6rem;
  color: var(--bt-ink);
}
.bt-404__body {
  color: var(--bt-ink-muted);
  margin: 0 0 1.4rem;
}

/* ---------- 13. Download page extras ---------- */
.bt-download-steps {
  counter-reset: bt-step;
  list-style: none;
  margin: 1.4rem 0;
  padding: 0;
}
.bt-download-steps li {
  position: relative;
  padding: 0.9rem 1rem 0.9rem 3.4rem;
  margin: 0 0 0.6rem;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-line);
  border-radius: var(--bt-radius);
}
.bt-download-steps li::before {
  counter-increment: bt-step;
  content: counter(bt-step);
  position: absolute;
  left: 14px;
  top: 0.9rem;
  width: 26px; height: 26px;
  display: flex; align-items: center; justify-content: center;
  background: var(--bt-brand);
  color: #FFFFFF;
  border-radius: 50%;
  font-family: var(--bt-font-serif);
  font-weight: 700;
  font-size: 14px;
}
.bt-promo-row {
  display: flex;
  flex-wrap: wrap;
  gap: 0.6rem;
  margin: 1rem 0 1.4rem;
  padding: 1rem 1.2rem;
  background: var(--bt-paper-warm);
  border: 1px solid var(--bt-line-strong);
  border-radius: var(--bt-radius);
}

/* ---------- 14. Mobile / responsive ---------- */
@media (max-width: 1180px) {
  :root {
    --bt-canvas-pad: 2.2rem;
    --bt-toc-w: 200px;
  }
}
@media (max-width: 1024px) {
  :root {
    --bt-sidebar-w: 280px;
    --bt-canvas-pad: 1.6rem;
  }
  .bt-article__body {
    grid-template-columns: 1fr;
    gap: 0;
  }
  .bt-toc {
    position: static;
    max-height: none;
    margin-top: 1.6rem;
    padding: 1rem;
    border-left: 0;
    border: 1px solid var(--bt-line);
    border-radius: var(--bt-radius);
    background: var(--bt-paper-warm);
  }
  .bt-footer__cols {
    grid-template-columns: 1fr 1fr 1fr;
  }
  .bt-footer__col--brand {
    grid-column: 1 / -1;
  }
}
@media (max-width: 768px) {
  body { font-size: 15.5px; }
  .bt-header__bar { padding: 0 0.6rem; }
  .bt-brand__title { display: none; }
  .bt-header__center { display: none; }
  .bt-header__bar { grid-template-columns: auto 1fr auto; }

  .bt-shell { display: block; }
  .bt-canvas { display: block; }
  .bt-sidebar {
    position: fixed;
    top: 0; left: 0;
    width: 84vw;
    max-width: 320px;
    height: 100vh;
    z-index: var(--bt-z-mobile-overlay);
    transform: translateX(-100%);
    transition: transform .2s ease;
    box-shadow: var(--bt-shadow-lg);
  }
  .bt-sidebar.is-open { transform: translateX(0); }
  .bt-nav-open::after {
    content: "";
    position: fixed;
    inset: 0;
    background: rgba(31,27,23,0.45);
    z-index: calc(var(--bt-z-mobile-overlay) - 1);
  }

  .bt-main {
    padding: 1rem 1rem 3rem;
  }
  .bt-article__h1 { font-size: 1.85rem; }
  .bt-cover { padding: 1.8rem 1.1rem 1.6rem; }
  .bt-cover__title { font-size: 1.85rem; }
  .bt-prose h2 { font-size: 1.45rem; }
  .bt-prose h3 { font-size: 1.18rem; }
  .bt-prose pre { padding: 0.8rem; font-size: 0.85rem; }

  .bt-prevnext { grid-template-columns: 1fr; gap: 0.6rem; }
  .bt-prevnext__placeholder { display: none; }
  .bt-prevnext__next { flex-direction: row; text-align: left; }
  .bt-prevnext__next .bt-prevnext__body { text-align: left; }

  .bt-footer__cols { grid-template-columns: 1fr 1fr; }
  .bt-footer__col--brand { grid-column: 1 / -1; }
  .bt-footer__bottom { flex-direction: column; }
  .bt-footer__risk { text-align: left; max-width: 100%; }

  .bt-cover__cta { flex-direction: column; align-items: stretch; }
  .bt-cover__cta .bt-btn { justify-content: center; }
}
@media (max-width: 480px) {
  .bt-footer__cols { grid-template-columns: 1fr; }
  .bt-cover__title { font-size: 1.55rem; }
  .bt-article__h1 { font-size: 1.6rem; }
  .bt-article__chap { font-size: 0.95rem; }
}

/* ---------- 15. Print ---------- */
@media print {
  .bt-header, .bt-sidebar, .bt-footer, .bt-toc, .bt-prevnext, .bt-pager,
  .bt-icon-btn, .bt-lang { display: none !important; }
  .bt-shell { display: block; }
  .bt-canvas { display: block; }
  .bt-main { max-width: 100%; padding: 0; }
  .bt-article__body { grid-template-columns: 1fr; }
  body { background: #FFFFFF; color: #000000; font-size: 11pt; }
  a { color: #000000; text-decoration: underline; }
  .bt-prose pre { white-space: pre-wrap; word-break: break-word; }
}

/* ---------- 16. Misc utilities ---------- */
.bt-only-mobile { display: none; }
@media (max-width: 768px) { .bt-only-mobile { display: inline-flex; } }
.bt-only-desktop { display: inline-flex; }
@media (max-width: 768px) { .bt-only-desktop { display: none; } }

/* visited tweak — Rust Book uses subtle different shade */
.bt-prose a:visited { color: var(--bt-visited); }
.bt-postlist__title:visited { color: var(--bt-ink-muted); }

/* line-numbered ordered list — semantic boost */
.bt-prose ol > li { padding-left: 0.2rem; }
.bt-prose ol > li::marker {
  color: var(--bt-brand);
  font-weight: 700;
  font-family: var(--bt-font-serif);
}

/* heading anchor (added by addHeadingAnchors filter) */
.bt-prose h2 .header-anchor,
.bt-prose h3 .header-anchor,
.bt-prose h4 .header-anchor {
  margin-left: 6px;
  color: var(--bt-ink-soft);
  text-decoration: none;
  font-weight: 400;
  opacity: 0;
  transition: opacity .15s;
}
.bt-prose h2:hover .header-anchor,
.bt-prose h3:hover .header-anchor,
.bt-prose h4:hover .header-anchor { opacity: 1; }

/* selection in dark mode */
[data-theme="dark"] ::selection {
  background: var(--bt-brand-strong);
  color: #FFFFFF;
}

/* sidebar scrollbar */
.bt-sidebar::-webkit-scrollbar { width: 6px; }
.bt-sidebar::-webkit-scrollbar-thumb { background: var(--bt-line-strong); border-radius: 3px; }
.bt-sidebar::-webkit-scrollbar-track { background: transparent; }
.bt-toc::-webkit-scrollbar { width: 4px; }
.bt-toc::-webkit-scrollbar-thumb { background: var(--bt-line-strong); border-radius: 2px; }

/* body class is-home toggles cover treatment if needed */
.is-home .bt-article { padding-top: 0.4rem; }

/* small inline kbd-ish */
.bt-prose kbd {
  display: inline-block;
  padding: 1px 6px;
  font-family: var(--bt-font-mono);
  font-size: 12px;
  background: var(--bt-paper-card);
  border: 1px solid var(--bt-line-strong);
  border-radius: 3px;
  box-shadow: inset 0 -1px 0 var(--bt-line);
  color: var(--bt-ink);
}

/* small badge near sidebar headings (chapter count) */
.bt-sidebar__badge {
  display: inline-block;
  margin-left: auto;
  padding: 0 5px;
  font-size: 10.5px;
  background: var(--bt-paper-warm);
  color: var(--bt-ink-soft);
  border-radius: 8px;
}