/* ===== Theme ===== */
:root {
  --bg: #0b0f17;
  --bg-soft: #131a26;
  --bg-card: #182030;
  --bg-card-hover: #1f2940;
  --border: #2a3349;
  --text: #e6e9ef;
  --text-dim: #8b95a8;
  --text-soft: #b7bfce;
  --accent: #7c9eff;
  --accent-2: #4cd6c1;
  --accent-3: #ff8c66;
  --accent-4: #ffd166;
  --success: #4ade80;
  --warn: #fbbf24;
  --danger: #f87171;
  --code-bg: #0e1421;
  --shadow: 0 6px 24px rgba(0,0,0,0.35);
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Inter", "Roboto", system-ui, sans-serif;
  line-height: 1.65;
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
}

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

code, pre, .pseudo {
  font-family: "JetBrains Mono", "Fira Code", ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
  font-size: 0.92em;
}

/* ===== Layout ===== */
.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 24px;
}

.topbar {
  position: sticky; top: 0; z-index: 50;
  background: rgba(11,15,23,0.85);
  backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--border);
}
.topbar .container {
  display: flex; align-items: center; justify-content: space-between;
  padding-top: 14px; padding-bottom: 14px;
}
.brand {
  display: flex; align-items: center; gap: 12px;
  font-weight: 700; color: var(--text); font-size: 18px;
}
.brand-mark {
  width: 28px; height: 28px; border-radius: 8px;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  display: inline-flex; align-items: center; justify-content: center;
  color: #0b0f17; font-weight: 900;
}
.nav-links { display: flex; gap: 22px; }
.nav-links a { color: var(--text-soft); font-size: 14px; }
.nav-links a:hover { color: var(--text); text-decoration: none; }

/* ===== Hero ===== */
.hero {
  padding: 72px 0 32px;
}
.hero h1 {
  font-size: clamp(32px, 5vw, 52px);
  margin: 0 0 16px;
  line-height: 1.1;
  background: linear-gradient(135deg, #fff 0%, var(--accent) 60%, var(--accent-2) 100%);
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
  font-weight: 800;
}
.hero p { font-size: 18px; color: var(--text-dim); max-width: 720px; margin: 0; }
.hero .tag {
  display: inline-block;
  font-size: 12px; letter-spacing: 0.08em; text-transform: uppercase;
  color: var(--accent-2); margin-bottom: 18px;
}

/* ===== Categories ===== */
.category {
  margin: 56px 0 12px;
}
.category-title {
  font-size: 14px; letter-spacing: 0.18em; text-transform: uppercase;
  color: var(--accent-2); margin: 0 0 6px;
}
.category-sub { color: var(--text-dim); margin: 0 0 18px; font-size: 14px; }

.cards {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 16px;
}
.card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 22px;
  transition: transform .15s ease, background .15s ease, border-color .15s ease;
  cursor: pointer;
  display: block;
  color: var(--text);
  text-decoration: none;
  position: relative;
  overflow: hidden;
}
.card:hover {
  background: var(--bg-card-hover);
  border-color: var(--accent);
  transform: translateY(-2px);
  text-decoration: none;
}
.card h3 {
  margin: 0 0 8px; font-size: 18px;
}
.card p {
  margin: 0; color: var(--text-dim); font-size: 14px;
}
.card .badge {
  display: inline-block;
  font-size: 11px;
  padding: 3px 8px;
  border-radius: 12px;
  background: rgba(124,158,255,0.15);
  color: var(--accent);
  margin-bottom: 10px;
  letter-spacing: 0.05em;
}
.card .arrow {
  position: absolute; top: 20px; right: 22px;
  color: var(--text-dim); transition: transform .15s ease, color .15s ease;
}
.card:hover .arrow { transform: translateX(3px); color: var(--accent); }

/* ===== Topic page ===== */
.topic-layout {
  display: grid;
  grid-template-columns: 240px 1fr;
  gap: 40px;
  padding: 32px 0 80px;
}
@media (max-width: 900px) {
  .topic-layout { grid-template-columns: 1fr; }
  .sidebar { position: static !important; }
}
.sidebar {
  position: sticky; top: 80px; align-self: start;
  font-size: 14px;
}
.sidebar h4 {
  font-size: 11px; letter-spacing: 0.18em; text-transform: uppercase;
  color: var(--text-dim); margin: 0 0 10px;
}
.sidebar ul { list-style: none; padding: 0; margin: 0 0 24px; }
.sidebar li { margin: 0; }
.sidebar a {
  display: block; padding: 6px 10px; border-radius: 6px;
  color: var(--text-soft); text-decoration: none;
  border-left: 2px solid transparent;
}
.sidebar a:hover { background: var(--bg-soft); color: var(--text); }
.sidebar a.active {
  background: var(--bg-soft); color: var(--accent);
  border-left-color: var(--accent);
}
.sidebar .back {
  display: inline-block; margin-bottom: 16px; font-size: 13px; color: var(--text-dim);
}

.topic-content { min-width: 0; }
.topic-content h1 {
  font-size: clamp(28px, 4vw, 40px);
  margin: 0 0 6px;
  line-height: 1.2;
}
.topic-content .lead {
  color: var(--text-dim); font-size: 16px; margin: 0 0 24px;
}
.topic-content h2 {
  font-size: 22px;
  margin: 40px 0 12px;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--border);
}
.topic-content h3 { font-size: 17px; margin: 24px 0 8px; }
.topic-content p, .topic-content li { color: var(--text-soft); }
.topic-content strong { color: var(--text); }

.topic-content ul, .topic-content ol { padding-left: 22px; }
.topic-content li { margin: 4px 0; }

/* ===== Boxes / Callouts ===== */
.callout {
  border-radius: 10px;
  padding: 14px 18px;
  margin: 16px 0;
  background: var(--bg-soft);
  border-left: 3px solid var(--accent);
}
.callout.intuition { border-left-color: var(--accent-2); }
.callout.warn { border-left-color: var(--warn); }
.callout.tip { border-left-color: var(--accent-3); }
.callout.exam { border-left-color: var(--accent-4); background: rgba(255,209,102,0.04); }
.callout h4 {
  margin: 0 0 6px; font-size: 13px; letter-spacing: 0.08em; text-transform: uppercase;
  color: var(--text);
}

/* ===== Code blocks ===== */
pre.pseudo, .topic-content pre {
  background: var(--code-bg);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 16px 18px;
  overflow-x: auto;
  line-height: 1.55;
  font-size: 13.5px;
  color: var(--text);
}
.kw { color: var(--accent); font-weight: 600; }
.fn { color: var(--accent-2); }
.cm { color: #6c7891; font-style: italic; }
.str { color: var(--accent-3); }
.num { color: var(--accent-4); }

/* ===== Tables ===== */
table {
  border-collapse: collapse; width: 100%;
  margin: 12px 0;
  font-size: 14px;
}
th, td {
  border: 1px solid var(--border);
  padding: 8px 12px;
  text-align: left;
}
th {
  background: var(--bg-soft); color: var(--text);
  font-weight: 600;
}
td { color: var(--text-soft); }
tr:nth-child(even) td { background: rgba(255,255,255,0.015); }

/* ===== Animation containers ===== */
.viz {
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 20px;
  margin: 16px 0;
}
.viz-canvas {
  width: 100%;
  background: var(--code-bg);
  border-radius: 8px;
  border: 1px solid var(--border);
  display: block;
}
.viz-controls {
  display: flex; gap: 8px; flex-wrap: wrap; margin-top: 14px;
  align-items: center;
}
.btn {
  background: var(--accent); color: #0b0f17;
  border: none; padding: 8px 16px; border-radius: 8px;
  font-weight: 600; font-size: 13px; cursor: pointer;
  font-family: inherit;
  transition: filter .12s ease;
}
.btn:hover { filter: brightness(1.1); }
.btn.secondary { background: var(--bg-card); color: var(--text); border: 1px solid var(--border); }
.btn:disabled { opacity: 0.4; cursor: not-allowed; }
.viz-status {
  margin-left: auto;
  font-size: 13px; color: var(--text-dim);
}
.viz-status b { color: var(--text); }
input[type="range"] {
  accent-color: var(--accent);
}
input[type="number"], input[type="text"], select {
  background: var(--code-bg); color: var(--text); border: 1px solid var(--border);
  border-radius: 6px; padding: 5px 8px; font-family: inherit; font-size: 13px;
}

/* ===== Diagrams ===== */
.diagram { width: 100%; }
.muted-text { fill: var(--text-dim); }
.accent-text { fill: var(--accent); }

/* Footer */
.footer {
  border-top: 1px solid var(--border);
  padding: 30px 0 50px;
  color: var(--text-dim);
  font-size: 13px;
  margin-top: 60px;
}

/* Math (KaTeX safety) */
.math-inline { padding: 0 2px; }