/* Math 53 Review — dark theme, Claude-web-app-inspired */

:root {
  --bg: #0f1419;
  --bg-elev: #161b22;
  --bg-side: #0b1014;
  --border: #21262d;
  --border-soft: #1a1f26;
  --text: #e6edf3;
  --text-dim: #9ba6b2;
  --text-mute: #6e7681;
  --accent: #7dd3fc;
  --accent-hover: #bae6fd;
  --accent-soft: rgba(125, 211, 252, 0.12);
  --green: #86efac;
  --amber: #fcd34d;
  --red: #fca5a5;
  --violet: #c4b5fd;
  --code-bg: #11161c;
  --font-sans: "Inter", ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  --font-mono: "JetBrains Mono", ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
}

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--font-sans);
  font-size: 16px;
  line-height: 1.7;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

.layout {
  display: grid;
  grid-template-columns: 300px 1fr;
  min-height: 100vh;
}

.sidebar {
  background: var(--bg-side);
  border-right: 1px solid var(--border);
  padding: 1.75rem 1.25rem 3rem;
  overflow-y: auto;
  position: sticky;
  top: 0;
  height: 100vh;
}

.sidebar .brand {
  display: block;
  font-weight: 600;
  font-size: 0.95rem;
  color: var(--text);
  text-decoration: none;
  padding-bottom: 1rem;
  border-bottom: 1px solid var(--border-soft);
  margin-bottom: 1rem;
}
.sidebar .brand .sub {
  display: block;
  color: var(--text-mute);
  font-weight: 400;
  font-size: 0.78rem;
  margin-top: 0.15rem;
}

.sidebar .part {
  color: var(--text-mute);
  text-transform: uppercase;
  font-size: 0.7rem;
  letter-spacing: 0.08em;
  margin: 1.25rem 0 0.35rem;
  font-weight: 600;
}

.sidebar ul {
  list-style: none;
  padding: 0;
  margin: 0 0 0.5rem;
}
.sidebar li { margin: 0; }
.sidebar .chapter {
  color: var(--text-dim);
  font-weight: 500;
  font-size: 0.82rem;
  padding: 0.35rem 0 0.15rem;
  margin-top: 0.25rem;
}
.sidebar a {
  color: var(--text-dim);
  text-decoration: none;
  display: block;
  padding: 0.2rem 0.55rem;
  border-radius: 5px;
  font-size: 0.84rem;
  transition: background 0.1s, color 0.1s;
  line-height: 1.35;
}
.sidebar a:hover {
  background: var(--accent-soft);
  color: var(--accent-hover);
}
.sidebar a.active {
  background: var(--accent-soft);
  color: var(--accent);
  font-weight: 500;
}

.main {
  padding: 2.5rem 3rem 6rem;
  max-width: 820px;
}

.crumbs {
  font-size: 0.8rem;
  color: var(--text-mute);
  margin-bottom: 1rem;
}
.crumbs a { color: var(--text-dim); text-decoration: none; }
.crumbs a:hover { color: var(--accent); }

h1, h2, h3, h4 {
  color: var(--text);
  line-height: 1.3;
  font-weight: 600;
}
h1 { font-size: 1.85rem; margin: 0 0 0.25rem; letter-spacing: -0.01em; }
h2 { font-size: 1.3rem; margin: 2.25rem 0 0.6rem; }
h3 { font-size: 1.05rem; margin: 1.5rem 0 0.4rem; color: var(--text-dim); }

.subtitle {
  color: var(--text-mute);
  font-size: 0.95rem;
  margin-top: 0;
  margin-bottom: 2rem;
}

p { margin: 0.6rem 0 1rem; }

a { color: var(--accent); text-decoration: none; }
a:hover { color: var(--accent-hover); text-decoration: underline; }

code {
  font-family: var(--font-mono);
  background: var(--code-bg);
  border: 1px solid var(--border-soft);
  border-radius: 4px;
  padding: 0.05rem 0.35rem;
  font-size: 0.88em;
}

pre {
  background: var(--code-bg);
  border: 1px solid var(--border-soft);
  border-radius: 8px;
  padding: 0.9rem 1rem;
  overflow-x: auto;
  font-size: 0.87rem;
}
pre code { background: transparent; border: none; padding: 0; }

ul, ol { padding-left: 1.3rem; }
li { margin: 0.2rem 0; }

hr { border: none; border-top: 1px solid var(--border-soft); margin: 2.5rem 0; }

.callout {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.85rem 1rem;
  margin: 1rem 0;
  background: var(--bg-elev);
}
.callout .label {
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-weight: 600;
  color: var(--text-mute);
  margin-bottom: 0.35rem;
}
.callout.def { border-left: 3px solid var(--accent); }
.callout.def .label { color: var(--accent); }
.callout.note { border-left: 3px solid var(--violet); }
.callout.note .label { color: var(--violet); }
.callout.warn { border-left: 3px solid var(--amber); }
.callout.warn .label { color: var(--amber); }
.callout.example { border-left: 3px solid var(--green); }
.callout.example .label { color: var(--green); }
.callout.thm { border-left: 3px solid var(--accent); background: #111821; }
.callout.thm .label { color: var(--accent); }

.callout > :last-child { margin-bottom: 0; }
.callout p:first-of-type { margin-top: 0; }

.problems { margin: 2rem 0; }
.problem {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1rem 1.2rem;
  margin: 0.8rem 0;
  background: var(--bg-elev);
}
.problem .tags {
  display: flex;
  gap: 0.5rem;
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-mute);
  margin-bottom: 0.4rem;
  font-weight: 600;
}
.problem .tag.easy { color: var(--green); }
.problem .tag.medium { color: var(--amber); }
.problem .tag.hard { color: var(--red); }

.problem .prompt { margin: 0.3rem 0 0.6rem; }
.problem details { margin-top: 0.35rem; }
.problem summary {
  cursor: pointer;
  color: var(--accent);
  font-size: 0.85rem;
  padding: 0.25rem 0;
  user-select: none;
}
.problem summary:hover { color: var(--accent-hover); }
.problem .hint, .problem .solution {
  margin-top: 0.3rem;
  padding: 0.55rem 0.85rem;
  background: #0d1318;
  border: 1px solid var(--border-soft);
  border-radius: 6px;
  font-size: 0.92rem;
}

figure {
  margin: 1.2rem 0;
  text-align: center;
}
figure svg {
  max-width: 100%;
  background: var(--bg-elev);
  border: 1px solid var(--border-soft);
  border-radius: 8px;
  padding: 0.4rem;
}
figcaption {
  font-size: 0.82rem;
  color: var(--text-mute);
  margin-top: 0.4rem;
}

.hero {
  padding: 1.5rem 0 2rem;
  border-bottom: 1px solid var(--border-soft);
  margin-bottom: 2rem;
}
.hero h1 { font-size: 2.1rem; margin-bottom: 0.4rem; }
.hero p.lede { color: var(--text-dim); font-size: 1.05rem; max-width: 60ch; }

.toc-grid { display: grid; gap: 1rem; }
.toc-part {
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--bg-elev);
  padding: 1.1rem 1.3rem;
}
.toc-part h2 {
  font-size: 1.05rem;
  margin: 0 0 0.35rem;
  color: var(--accent);
}
.toc-part p.desc {
  color: var(--text-mute);
  font-size: 0.88rem;
  margin: 0 0 0.65rem;
}
.toc-part ul {
  list-style: none;
  padding: 0;
  margin: 0;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.15rem 0.8rem;
}
.toc-part li { margin: 0; }
.toc-part a {
  display: block;
  padding: 0.25rem 0;
  font-size: 0.9rem;
  color: var(--text-dim);
}
.toc-part a:hover { color: var(--accent); text-decoration: none; }

.katex { font-size: 1.02em; }
.katex-display { margin: 1rem 0; overflow-x: auto; overflow-y: hidden; }

.prev-next {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  margin: 3rem 0 1rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border-soft);
}
.prev-next a {
  flex: 1;
  padding: 0.75rem 1rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--bg-elev);
  font-size: 0.88rem;
}
.prev-next a:hover { border-color: var(--accent); text-decoration: none; }
.prev-next a.next { text-align: right; }
.prev-next .dir { color: var(--text-mute); font-size: 0.72rem; text-transform: uppercase; letter-spacing: 0.08em; display: block; margin-bottom: 0.15rem; }

@media (max-width: 900px) {
  .layout { grid-template-columns: 1fr; }
  .sidebar {
    position: static;
    height: auto;
    border-right: none;
    border-bottom: 1px solid var(--border);
  }
  .main { padding: 1.5rem 1.25rem 4rem; }
  .toc-part ul { grid-template-columns: 1fr; }
}