:root {
  --sw:        210px;
  --th:         44px;

  /* surface */
  --bg:        #111113;
  --bg-side:   #0d0d0f;
  --bg-code:   #0a0a0c;
  --border:    #1e1e24;

  /* text */
  --text:      #a8a8bc;
  --muted:     #484860;
  --heading:   #ececf4;
  --accent:    #818cf8;
  --em:        #8888a8;

  /* nav */
  --nav-group: #42425a;
  --nav-link:  #7878a0;
  --nav-hover: #aeaece;
  --nav-active:#f0f0fa;
  --nav-dim:   #48486a;

  /* syntax */
  --code-inline: #9db8c8;
  --syntax-kw:   #a898d8;
  --syntax-val:  #d8a888;

  /* callouts */
  --note-border: #506880;
  --note-label:  #6888a0;
  --warn-border: #807040;
  --warn-label:  #908050;

  --font:      "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  --mono:      "SFMono-Regular", Consolas, "Liberation Mono", monospace;
}

*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }

body {
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.75;
  font-size: 15px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

/* ── Sidebar ── */
.sidebar {
  position: fixed;
  top: 0; left: 0;
  width: var(--sw);
  height: 100vh;
  background: var(--bg-side);
  border-right: 1px solid var(--border);
  overflow-y: auto;
  overflow-x: hidden;
  display: flex;
  flex-direction: column;
  z-index: 200;
  transition: transform 0.22s ease;
}

.sidebar-logo {
  display: flex;
  align-items: center;
  gap: 0.55rem;
  padding: 0 0.9rem;
  height: var(--th);
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
  text-decoration: none;
  color: inherit;
}
.sidebar-logo svg { flex-shrink: 0; color: var(--heading); opacity: 0.9; }
.sidebar-logo-text {
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--heading);
  letter-spacing: -0.01em;
  white-space: nowrap;
}

.sidebar-nav {
  padding: 0.4rem 0 3rem;
}

.nav-group {
  padding: 1.1rem 1rem 0.3rem;
  font-size: 0.58rem;
  font-weight: 600;
  text-transform: uppercase;
  color: var(--nav-group);
  user-select: none;
}

.sidebar-nav a,
.nav-soon {
  display: block;
  margin: 1px 6px;
  padding: 0.3rem 0.8rem;
  font-size: 0.84rem;
  font-weight: 500;
  border-radius: 5px;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.sidebar-nav a {
  color: var(--nav-link);
  text-decoration: none;
  transition: color 0.14s, background 0.14s;
}
.sidebar-nav a:hover {
  color: var(--nav-hover);
  background: rgba(255,255,255,0.05);
}
.sidebar-nav a.active {
  color: var(--nav-active);
  background: rgba(255,255,255,0.09);
}

.nav-soon {
  color: var(--nav-dim);
  cursor: default;
}

/* ── Sidebar accordion ── */
.nav-accordion-btn {
  display: flex;
  align-items: center;
  justify-content: space-between;
  width: calc(100% - 12px);
  margin: 1px 6px;
  padding: 0.3rem 0.8rem;
  background: none;
  border: none;
  border-radius: 5px;
  font-family: var(--font);
  font-size: 0.84rem;
  font-weight: 500;
  color: var(--nav-link);
  cursor: pointer;
  text-align: left;
  transition: color 0.14s, background 0.14s;
}
.nav-accordion-btn:hover {
  color: var(--nav-hover);
  background: rgba(255,255,255,0.05);
}
.nav-accordion-icon {
  flex-shrink: 0;
  color: var(--nav-dim);
  transition: transform 0.2s ease;
}
.nav-accordion.open .nav-accordion-icon { transform: rotate(90deg); }

.nav-accordion-items {
  display: none;
  flex-direction: column;
}
.nav-accordion.open .nav-accordion-items { display: flex; }

.nav-child {
  padding-left: 1.5rem !important;
  font-size: 0.8rem !important;
}

/* ── Topbar ── */
.topbar {
  position: sticky;
  top: 0;
  z-index: 100;
  height: var(--th);
  display: flex;
  align-items: center;
  gap: 0.75rem;
  padding: 0 1.25rem;
  background: var(--bg);
  border-bottom: 1px solid var(--border);
  margin-left: var(--sw);
  transition: margin-left 0.22s ease;
}

.toggle-btn {
  flex-shrink: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 26px; height: 26px;
  background: none;
  border: none;
  border-radius: 4px;
  color: var(--muted);
  cursor: pointer;
  transition: color 0.15s, background 0.15s;
}
.toggle-btn:hover { color: var(--heading); background: rgba(255,255,255,0.06); }

.topbar-crumb {
  font-size: 0.78rem;
  color: var(--muted);
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

/* ── Main ── */
.main { margin-left: var(--sw); transition: margin-left 0.22s ease; }

.content {
  max-width: 1100px;
  margin: 0 auto;
  padding: 3.5rem 4rem 6rem;
}

/* ── Collapsed ── */
body.sidebar-off .sidebar  { transform: translateX(-100%); }
body.sidebar-off .main     { margin-left: 0; }
body.sidebar-off .topbar   { margin-left: 0; }

/* ── Intro page ── */
.intro-page {
  min-height: calc(100vh - var(--th));
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 4rem 2rem 5rem;
  background:
    radial-gradient(ellipse 70% 80% at 50% 0%, rgba(129,140,248,0.07) 0%, transparent 65%);
}

.overview {
  width: 100%;
  max-width: 480px;
  text-align: center;
}

.overview-eyebrow {
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--accent);
  opacity: 0.8;
  margin-bottom: 1.25rem;
}

.overview-title {
  font-size: clamp(2.6rem, 5vw, 3.5rem);
  font-weight: 700;
  color: var(--heading);
  letter-spacing: -0.05em;
  line-height: 1.06;
  margin-bottom: 2rem;
}

.overview-desc {
  font-size: 0.975rem;
  color: var(--text);
  line-height: 1.8;
  max-width: 400px;
  margin: 0 auto;
}

.intro-start {
  margin-top: 1.75rem;
}

.intro-start a {
  font-size: 0.9rem;
  font-weight: 500;
  color: var(--accent);
  text-decoration: none;
  border-bottom: 1px solid rgba(129, 140, 248, 0.35);
  padding-bottom: 0.1rem;
  transition: color 0.14s, border-color 0.14s;
}

.intro-start a:hover {
  color: var(--heading);
  border-color: var(--accent);
}

.overview-divider {
  border: none;
  border-top: 1px solid var(--border);
  margin: 2.5rem auto;
  width: 60%;
}

.overview-attribution {
  font-size: 0.83rem;
  color: var(--muted);
  line-height: 1.9;
}
.overview-attribution strong {
  color: var(--text);
  font-weight: 500;
}

/* ── Typography (chapter pages) ── */
h1 { font-size: 1.8rem; color: var(--heading); font-weight: 700; margin-bottom: 0.35rem; line-height: 1.2; letter-spacing: -0.03em; }
h2 { font-size: 1.15rem; color: var(--heading); font-weight: 600; margin: 2.25rem 0 0.5rem; letter-spacing: -0.01em; }
h3 { font-size: 0.95rem; color: var(--heading); font-weight: 600; margin: 1.5rem 0 0.35rem; }

.page-meta { font-size: 0.75rem; color: var(--muted); margin-bottom: 1.75rem; padding-bottom: 1.75rem; border-bottom: 1px solid var(--border); }

p { margin-bottom: 0.9rem; }
p:last-child { margin-bottom: 0; }

a { color: var(--text); text-decoration: underline; text-underline-offset: 3px; text-decoration-color: var(--border); }
a:hover { color: var(--heading); text-decoration-color: var(--muted); }

ul, ol { padding-left: 1.35rem; margin-bottom: 0.9rem; }
li { margin-bottom: 0.2rem; }

strong { color: var(--heading); font-weight: 600; }
em     { color: var(--em); font-style: normal; }

hr { border: none; border-top: 1px solid var(--border); margin: 2rem 0; }

/* ── Code ── */
code {
  font-family: var(--mono);
  font-size: 0.82em;
  background: var(--bg-code);
  color: var(--code-inline);
  padding: 0.12em 0.35em;
  border-radius: 3px;
  border: 1px solid var(--border);
}
pre {
  background: var(--bg-code);
  border: 1px solid var(--border);
  border-radius: 5px;
  padding: 1.1rem 1.35rem;
  overflow-x: auto;
  margin: 1.1rem 0;
}
pre code { background: none; border: none; padding: 0; font-size: 0.85rem; color: var(--text); line-height: 1.65; }
.comment { color: var(--muted); }
.kw  { color: var(--syntax-kw); }
.val { color: var(--syntax-val); }

/* ── Callout ── */
.callout {
  background: rgba(255,255,255,0.02);
  border-radius: 0 5px 5px 0;
  padding: 0.85rem 1.1rem;
  margin: 1.35rem 0;
}
.callout.note { border-color: var(--note-border); background: rgba(50,80,110,0.08); }
.callout.warn { border-color: var(--warn-border); background: rgba(120,100,40,0.08); }
.callout-label {
  font-size: 0.68rem; font-weight: 700; text-transform: uppercase;
  color: var(--muted); margin-bottom: 0.35rem;
}
.callout.note .callout-label { color: var(--note-label); }
.callout.warn .callout-label { color: var(--warn-label); }

/* ── Placeholder ── */
.placeholder {
  border: 1px dashed var(--border);
  border-radius: 5px;
  padding: 1.75rem;
  text-align: center;
  color: var(--muted);
  font-size: 0.82rem;
  margin: 1.35rem 0;
}

/* ── Prev/Next ── */
.page-nav {
  display: flex;
  justify-content: space-between;
  gap: 0.75rem;
  margin-top: 4rem;
  padding-top: 1.75rem;
  border-top: 1px solid var(--border);
}
.page-nav a {
  font-size: 0.82rem;
  color: var(--muted);
  padding: 0.5rem 0.9rem;
  border: 1px solid var(--border);
  border-radius: 4px;
  text-decoration: none;
  transition: color 0.15s, border-color 0.15s;
}
.page-nav a:hover { color: var(--heading); border-color: var(--muted); }
.page-nav a.next  { margin-left: auto; }

/* ── Overlay ── */
.overlay { display: none; position: fixed; inset: 0; background: rgba(0,0,0,0.55); z-index: 150; }
.overlay.visible { display: block; }

/* ── KaTeX ── */
.katex-display {
  background: rgba(255,255,255,0.025);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 1.1rem 1.5rem;
  margin: 1.35rem 0;
  overflow-x: auto;
}
.katex { color: var(--heading); }
.katex-display > .katex { color: var(--heading); }

/* ── Math table ── */
.math-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.88rem;
  margin: 1.25rem 0;
}
.math-table th {
  text-align: left;
  padding: 0.5rem 1rem;
  font-weight: 600;
  color: var(--heading);
  border-bottom: 1px solid var(--border);
  font-size: 0.82rem;
}
.math-table td {
  padding: 0.45rem 1rem;
  color: var(--text);
  border-bottom: 1px solid rgba(255,255,255,0.04);
}
.math-table tbody tr:last-child td { border-bottom: none; }
.math-table tbody tr:hover td { background: rgba(255,255,255,0.03); }

/* ── Graph Coloring Demo ── */
.demo-section {
  margin: 2rem 0;
  padding: 1.6rem 2rem 1.4rem;
  background: rgba(255,255,255,0.022);
  border: 1px solid var(--border);
}

.demo-caption {
  font-size: 0.79rem;
  font-style: italic;
  font-family: Georgia, "Times New Roman", serif;
  color: var(--muted);
  line-height: 1.65;
  margin: 0.85rem 0 0;
  padding-top: 0.85rem;
  border-top: 1px solid var(--border);
}
.demo-caption strong { color: var(--text); font-style: normal; font-weight: 600; }

.demo-inner { width: 100%; }

.demo-stats {
  display: flex;
  gap: 2rem;
  margin-bottom: 1rem;
  padding-bottom: 0.7rem;
  border-bottom: 1px solid var(--border);
}

.demo-stat-value {
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--heading);
  line-height: 1;
  font-variant-numeric: tabular-nums;
}

.demo-stat-label {
  font-size: 0.62rem;
  font-weight: 600;
  text-transform: uppercase;
  color: var(--muted);
  margin-top: 0.3rem;
}

.demo-eyebrow {
  display: inline-block;
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--muted);
  margin-bottom: 0.6rem;
}

.demo-title {
  font-size: 1.3rem;
  font-weight: 600;
  color: var(--heading);
  font-family: Georgia, "Times New Roman", serif;
  margin-bottom: 0.6rem;
  line-height: 1.3;
  letter-spacing: -0.01em;
}

.demo-desc {
  font-size: 0.88rem;
  color: var(--text);
  line-height: 1.75;
  max-width: 540px;
}

.demo-desc em { color: var(--heading); font-style: italic; font-weight: 400; }

.demo-body {
  display: flex;
  flex-direction: row;
  align-items: flex-start;
  gap: 2.5rem;
  margin-top: 0;
}

.demo-graph-wrap {
  flex: 1;
  line-height: 0;
}

.demo-graph-wrap svg {
  width: 100%;
  height: auto;
  display: block;
}

.demo-panel {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: 0.85rem;
  padding-top: 0.2rem;
}

.demo-progress {
  display: flex;
  gap: 4px;
}

.demo-pip {
  height: 2px;
  width: 28px;
  border-radius: 0;
  background: var(--border);
  transition: background 0.25s;
}
.demo-pip.active { background: var(--heading); }
.demo-pip.done   { background: var(--muted); }

.demo-msg {
  font-size: 0.875rem;
  line-height: 1.8;
  color: var(--text);
  min-height: 3rem;
}

.demo-actions {
  display: flex;
  gap: 0.6rem;
  align-items: center;
}

.demo-action-btn,
.demo-again-btn {
  display: inline-flex;
  align-items: center;
  background: transparent;
  border-radius: 2px;
  font-family: var(--font);
  font-size: 0.8rem;
  cursor: pointer;
  border: 1px solid;
}

.demo-action-btn {
  padding: 0.3rem 1rem;
  color: var(--heading);
  border-color: var(--muted);
  font-weight: 500;
  transition: background 0.15s, border-color 0.15s;
}
.demo-action-btn:hover  { background: rgba(255,255,255,0.06); border-color: var(--heading); }
.demo-action-btn:active { background: rgba(255,255,255,0.1); }

.demo-again-btn {
  padding: 0.3rem 0.9rem;
  color: var(--muted);
  border-color: var(--border);
  font-weight: 400;
  transition: color 0.15s, border-color 0.15s;
}
.demo-again-btn:hover { color: var(--text); border-color: var(--muted); }

/* ── Mobile ── */
@media (max-width: 720px) {
  .sidebar { transform: translateX(-100%); }
  .sidebar.open { transform: translateX(0); box-shadow: 8px 0 40px rgba(0,0,0,0.7); }
  body.sidebar-off .sidebar { transform: translateX(-100%); }
  .topbar { margin-left: 0 !important; }
  .main   { margin-left: 0 !important; }
  .content { padding: 1.75rem 1.15rem 4rem; }
  .intro-page { padding: 3rem 1.5rem 5rem; }
  .demo-section {
    margin: 1.75rem 0;
    padding: 1.25rem 1.15rem 1rem;
  }
  .demo-body {
    flex-direction: column;
    gap: 1.5rem;
  }
  .demo-graph-wrap {
    flex: unset;
    width: 100%;
  }
}