/* ── CSS Custom Properties ── */
:root {
  --bg: #fafaf9;
  --bg-sidebar: #f1f5f9;
  --bg-header: #0c4a6e;
  --bg-active: #dbeafe;
  --text: #1c1917;
  --text-muted: #64748b;
  --text-active: #1d4ed8;
  --text-header: #ffffff;
  --text-header-sub: #7dd3fc;
  --accent: #0369a1;
  --border: #e2e8f0;
  --part-header-bg: #e2e8f0;
  --part-header-text: #334155;
  --blockquote-bg: #e0f2fe;
  --blockquote-border: #0369a1;
  --info-card-bg: #e0f2fe;
  --warn-card-bg: #fff7ed;
  --sidebar-width: 268px;
}

[data-theme="dark"] {
  --bg: #0f172a;
  --bg-sidebar: #1e293b;
  --bg-header: #1e293b;
  --bg-active: #1d4ed8;
  --text: #cbd5e1;
  --text-muted: #64748b;
  --text-active: #bfdbfe;
  --text-header: #f1f5f9;
  --text-header-sub: #38bdf8;
  --accent: #38bdf8;
  --border: #334155;
  --part-header-bg: #334155;
  --part-header-text: #e2e8f0;
  --blockquote-bg: #1e3a5f;
  --blockquote-border: #38bdf8;
  --info-card-bg: #1e3a5f;
  --warn-card-bg: #2d1a0e;
}

/* ── Reset & Base ── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }
body {
  font-family: 'Be Vietnam Pro', 'Inter', system-ui, sans-serif;
  background: var(--bg);
  color: var(--text);
  font-size: 15px;
  line-height: 1.7;
  overflow-x: hidden;
  transition: background 0.2s, color 0.2s;
}

/* ── Layout ── */
.layout { display: flex; min-height: 100vh; }

/* ── Sidebar ── */
.sidebar {
  position: fixed;
  top: 0; left: 0;
  width: var(--sidebar-width);
  height: 100vh;
  overflow-y: auto;
  background: var(--bg-sidebar);
  border-right: 1px solid var(--border);
  padding: 16px 12px;
  z-index: 10;
  display: flex;
  flex-direction: column;
  gap: 4px;
  transition: transform 0.25s ease, background 0.2s;
}
.sidebar-brand {
  font-size: 12px;
  font-weight: 800;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  padding-bottom: 10px;
  border-bottom: 2px solid var(--accent);
  margin-bottom: 8px;
  text-decoration: none;
  display: block;
}
.part-group { margin-bottom: 4px; }
.part-toggle {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 6px 8px;
  background: var(--part-header-bg);
  color: var(--part-header-text);
  border-radius: 6px;
  cursor: pointer;
  font-size: 12px;
  font-weight: 700;
  border: none;
  width: 100%;
  text-align: left;
  transition: background 0.15s;
}
.part-toggle:hover { filter: brightness(0.95); }
.part-toggle .arrow { transition: transform 0.2s; font-size: 10px; }
.part-toggle.open .arrow { transform: rotate(90deg); }
.part-chapters { display: none; padding-left: 8px; margin-top: 3px; }
.part-chapters.open { display: flex; flex-direction: column; gap: 1px; }
.chapter-link {
  display: block;
  padding: 5px 8px;
  border-radius: 5px;
  color: var(--text);
  font-size: 12px;
  text-decoration: none;
  transition: background 0.1s;
  opacity: 0.85;
}
.chapter-link:hover { background: var(--bg-active); color: var(--accent); opacity: 1; }
.chapter-link.active {
  background: var(--bg-active);
  color: var(--text-active);
  font-weight: 600;
  opacity: 1;
}

/* ── Sticky Header ── */
.site-header {
  position: sticky;
  top: 0;
  background: var(--bg-header);
  color: var(--text-header);
  padding: 8px 20px;
  display: flex;
  align-items: center;
  gap: 10px;
  z-index: 5;
  min-height: 48px;
  transition: background 0.2s;
}
.header-breadcrumb { flex: 1; font-size: 13px; }
.header-breadcrumb .part-label { color: var(--text-header-sub); }
.header-breadcrumb .sep { color: #475569; margin: 0 4px; }
.header-breadcrumb .ch-label { font-weight: 700; }
.header-nav { display: flex; gap: 6px; align-items: center; }
.header-nav a {
  background: rgba(255,255,255,0.15);
  color: var(--text-header);
  padding: 3px 10px;
  border-radius: 4px;
  font-size: 12px;
  text-decoration: none;
  white-space: nowrap;
}
.header-nav a:hover { background: rgba(255,255,255,0.25); }
.theme-toggle {
  background: rgba(255,255,255,0.15);
  border: none;
  color: var(--text-header);
  padding: 4px 8px;
  border-radius: 4px;
  cursor: pointer;
  font-size: 14px;
  line-height: 1;
}
.theme-toggle:hover { background: rgba(255,255,255,0.25); }
.hamburger {
  display: none;
  background: none;
  border: none;
  color: var(--text-header);
  font-size: 20px;
  cursor: pointer;
  padding: 2px 6px;
  margin-right: 4px;
}

/* ── Main Content ── */
.main-content { margin-left: var(--sidebar-width); flex: 1; min-width: 0; }
.content-inner { max-width: 840px; margin: 0 auto; padding: 32px 32px 60px; }

/* ── Typography ── */
.content-inner h2 {
  font-size: 20px; font-weight: 800; color: var(--accent);
  margin-bottom: 16px; padding-bottom: 10px;
  border-bottom: 3px solid var(--accent);
}
.content-inner h3 { font-size: 16px; font-weight: 700; color: var(--accent); margin: 24px 0 10px; }
.content-inner h4 { font-size: 15px; font-weight: 600; margin: 14px 0 6px; }
.content-inner p { margin-bottom: 12px; }
.content-inner ul, .content-inner ol { margin: 10px 0 14px 22px; }
.content-inner li { margin-bottom: 6px; line-height: 1.6; }
.content-inner blockquote {
  border-left: 4px solid var(--blockquote-border);
  margin: 16px 0; padding: 12px 16px;
  background: var(--blockquote-bg);
  color: var(--accent);
  font-style: italic;
  border-radius: 0 8px 8px 0;
}
.content-inner table { width: 100%; border-collapse: collapse; margin: 16px 0; font-size: 14px; }
.content-inner th { background: var(--accent); color: #fff; padding: 10px 14px; text-align: left; }
.content-inner td { padding: 9px 14px; border-bottom: 1px solid var(--border); }
.content-inner tr:nth-child(even) td { background: var(--bg-sidebar); }
.content-inner .info-card {
  background: var(--info-card-bg); border: 1px solid var(--blockquote-border);
  border-radius: 10px; padding: 16px; margin: 14px 0;
}
.content-inner code { background: var(--bg-sidebar); padding: 2px 6px; border-radius: 4px; font-size: 13px; }
.content-inner pre { background: var(--bg-sidebar); padding: 16px; border-radius: 8px; overflow-x: auto; margin: 12px 0; }
.content-inner pre code { background: none; padding: 0; }

/* ── Bottom Nav ── */
.bottom-nav {
  border-top: 1px solid var(--border);
  padding: 16px 32px;
  display: flex;
  justify-content: space-between;
  background: var(--bg-sidebar);
  max-width: 840px; margin: 0 auto;
}
.bottom-nav a { color: var(--accent); text-decoration: none; font-size: 13px; }
.bottom-nav a:hover { text-decoration: underline; }

/* ── Homepage ── */
.homepage-hero { background: var(--bg-header); color: #fff; padding: 40px 32px; margin-left: var(--sidebar-width); }
.homepage-hero h1 { font-size: 28px; font-weight: 800; margin-bottom: 8px; }
.homepage-hero .subtitle { color: var(--text-header-sub); font-size: 15px; }
.stat-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(120px, 1fr)); gap: 12px; margin: 20px 0; }
.stat-card { background: var(--bg-sidebar); border: 1px solid var(--border); border-radius: 10px; padding: 16px; text-align: center; }
.stat-card .num { font-size: 28px; font-weight: 800; color: var(--accent); }
.stat-card .label { font-size: 12px; color: var(--text-muted); }
.chapter-card { background: var(--bg-sidebar); border: 1px solid var(--border); border-radius: 8px; padding: 12px 16px; display: flex; gap: 12px; align-items: flex-start; text-decoration: none; color: var(--text); transition: border-color 0.15s; margin-bottom: 8px; }
.chapter-card:hover { border-color: var(--accent); }
.chapter-card .ch-num { width: 32px; height: 32px; border-radius: 6px; background: var(--accent); color: #fff; font-size: 13px; font-weight: 800; display: flex; align-items: center; justify-content: center; flex-shrink: 0; }
.chapter-card .ch-title { font-size: 14px; font-weight: 600; color: var(--accent); }
.part-section { margin-bottom: 32px; }
.part-section h2 { font-size: 16px; font-weight: 800; color: var(--text-muted); text-transform: uppercase; letter-spacing: 0.06em; margin-bottom: 12px; padding-bottom: 6px; border-bottom: 1px solid var(--border); }

/* ── Overlay (mobile sidebar) ── */
.sidebar-overlay {
  display: none;
  position: fixed; inset: 0;
  background: rgba(0,0,0,0.5);
  z-index: 9;
}

/* ── Mobile ── */
@media (max-width: 767px) {
  .sidebar { transform: translateX(-100%); }
  .sidebar.open { transform: translateX(0); }
  .sidebar-overlay.open { display: block; }
  .hamburger { display: block; }
  .main-content { margin-left: 0; }
  .homepage-hero { margin-left: 0; }
  .content-inner { padding: 16px 16px 48px; }
  .bottom-nav { padding: 12px 16px; }
  table { display: block; overflow-x: auto; }
}