:root {
  --bg: #0f1419;
  --surface: #1a2332;
  --border: #2d3a4f;
  --text: #e8edf4;
  --muted: #8b9cb3;
  --accent: #5b9fd4;
  --accent-dim: #3d6f94;
  --good: #6bc9a8;
  --warn: #e8b86d;
  --font: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  --mono: ui-monospace, "Cascadia Code", "SF Mono", monospace;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
  padding: 0 1rem 2rem;
}

a { color: var(--accent); }

.site-header, .lesson-header {
  max-width: 900px;
  margin: 0 auto;
  padding: 1.5rem 0 1rem;
}

.site-header h1, .lesson-header h1 {
  margin: 0 0 0.25rem;
  font-size: 1.5rem;
  font-weight: 600;
}

.site-header p, .lesson-header p {
  margin: 0;
  color: var(--muted);
  font-size: 0.95rem;
}

.site-tagline {
  font-size: 1.05rem !important;
  color: var(--text) !important;
  max-width: 42rem;
}

.site-meta {
  margin-top: 0.5rem !important;
  font-size: 0.85rem !important;
}

.lesson-meta {
  font-size: 0.82rem !important;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  margin-bottom: 0.5rem !important;
}

.lesson-page {
  max-width: 900px;
  margin: 0 auto;
}

.lesson-section {
  margin-bottom: 1.75rem;
}

.lesson-section[hidden],
.glossary[hidden],
.lesson-nav[hidden],
.callout[hidden] {
  display: none !important;
}

.section-heading {
  margin: 0 0 0.75rem;
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--text);
}

.callout {
  max-width: 900px;
  margin: 0 auto 1.5rem;
  padding: 1rem 1.15rem;
  background: linear-gradient(135deg, rgba(91, 159, 212, 0.12), rgba(107, 201, 168, 0.08));
  border: 1px solid var(--accent-dim);
  border-radius: 8px;
}

.callout strong {
  display: block;
  margin-bottom: 0.35rem;
  color: var(--accent);
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.callout p {
  margin: 0;
  font-size: 0.95rem;
  line-height: 1.55;
}

.objective-list,
.takeaway-list,
.explore-list,
.external-list {
  margin: 0;
  padding-left: 1.25rem;
  display: flex;
  flex-direction: column;
  gap: 0.45rem;
  font-size: 0.95rem;
}

.external-list {
  list-style: none;
  padding-left: 0;
}

.external-list a {
  display: inline-block;
  padding: 0.4rem 0;
}

.video-caption {
  margin: 0 0 0.75rem;
  font-size: 0.92rem;
}

.video-caption .muted {
  color: var(--muted);
}

.video-embed {
  position: relative;
  width: 100%;
  padding-bottom: 56.25%;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  overflow: hidden;
}

.video-embed iframe {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  border: 0;
}

.video-embed--compact {
  padding-bottom: 50%;
  margin: 0.75rem 0;
}

.example-title {
  margin: 0 0 0.5rem;
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--good);
}

.formula-block {
  font-family: var(--mono);
  font-size: 0.88rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 0.85rem 1rem;
  margin: 0.5rem 0 0;
  white-space: pre-wrap;
  line-height: 1.6;
  overflow-x: auto;
}

.practice-prompt {
  margin: 0 0 0.75rem;
  font-size: 0.95rem;
  padding: 0.85rem 1rem;
  background: var(--surface);
  border-left: 3px solid var(--warn);
  border-radius: 0 6px 6px 0;
}

.practice-hint {
  font-size: 0.9rem;
  color: var(--muted);
}

.practice-hint summary {
  cursor: pointer;
  color: var(--accent);
  user-select: none;
}

.practice-hint p {
  margin: 0.5rem 0 0;
  color: var(--text);
}

.recap-grid {
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

.recap-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1rem 1.15rem;
}

.recap-card header {
  margin-bottom: 0.5rem;
}

.recap-id {
  font-size: 0.78rem;
  color: var(--accent);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.recap-card h3 {
  margin: 0.2rem 0 0;
  font-size: 1rem;
}

.recap-source {
  margin: 0.25rem 0 0;
  font-size: 0.82rem;
  color: var(--muted);
}

.recap-card > p {
  margin: 0.5rem 0 0;
  font-size: 0.92rem;
  color: var(--muted);
}

.recap-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem 1.25rem;
  margin-top: 0.75rem;
  align-items: center;
  font-size: 0.88rem;
}

.lesson-nav {
  max-width: 900px;
  margin: 2rem auto 0;
  padding-top: 1.25rem;
  border-top: 1px solid var(--border);
  display: grid;
  grid-template-columns: 1fr auto 1fr;
  gap: 1rem;
  align-items: center;
  font-size: 0.9rem;
}

.nav-prev { justify-self: start; }
.nav-home { justify-self: center; color: var(--muted); }
.nav-next { justify-self: end; }

.lesson-blurb {
  font-size: 0.85rem;
  color: var(--muted);
  line-height: 1.4;
}

.lesson-badge {
  display: inline-block;
  margin-top: 0.35rem;
  font-size: 0.72rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--accent);
  background: rgba(91, 159, 212, 0.12);
  padding: 0.2rem 0.45rem;
  border-radius: 4px;
  width: fit-content;
}

.site-footer {
  max-width: 900px;
  margin: 2rem auto 0;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  font-size: 0.85rem;
  color: var(--muted);
}

.site-footer p { margin: 0; }

#interactive .viz-app {
  margin-top: 0.5rem;
}

.back {
  display: inline-block;
  margin-bottom: 0.75rem;
  font-size: 0.9rem;
  text-decoration: none;
}

.lesson-grid {
  max-width: 900px;
  margin: 0 auto;
  display: grid;
  gap: 0.75rem;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
}

.lesson-card {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
  padding: 1rem 1.1rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  text-decoration: none;
  color: inherit;
  transition: border-color 0.15s;
}

.lesson-card:hover { border-color: var(--accent); }

.lesson-id {
  font-size: 0.8rem;
  color: var(--accent);
  font-weight: 600;
}

.lesson-title { font-size: 0.95rem; }

.viz-app {
  max-width: 900px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

.panel {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1rem 1.1rem;
}

.panel h2 {
  margin: 0 0 0.75rem;
  font-size: 1rem;
  font-weight: 600;
}

.panel p.hint {
  margin: 0 0 1rem;
  font-size: 0.88rem;
  color: var(--muted);
}

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem 1.5rem;
  margin-bottom: 1rem;
}

.control {
  flex: 1 1 160px;
  min-width: 140px;
}

.control label {
  display: block;
  font-size: 0.82rem;
  color: var(--muted);
  margin-bottom: 0.35rem;
}

.control output {
  float: right;
  font-family: var(--mono);
  font-size: 0.82rem;
  color: var(--accent);
}

input[type="range"] {
  width: 100%;
  accent-color: var(--accent);
}

.formula {
  font-family: var(--mono);
  font-size: 0.88rem;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 0.75rem 1rem;
  margin: 0.75rem 0;
  white-space: pre-wrap;
  line-height: 1.6;
}

.chart-wrap {
  position: relative;
  height: 280px;
  margin-top: 0.5rem;
}

.stats-row {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem 1.5rem;
  margin-top: 0.75rem;
  font-size: 0.88rem;
}

.stat {
  font-family: var(--mono);
}

.stat span { color: var(--muted); }

.glossary {
  max-width: 900px;
  margin: 1.5rem auto 0;
  padding: 1rem 1.1rem;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: 0.88rem;
}

.glossary h3 {
  margin: 0 0 0.5rem;
  font-size: 0.95rem;
}

.glossary dl {
  margin: 0;
  display: grid;
  gap: 0.35rem;
}

.glossary dt {
  font-family: var(--mono);
  color: var(--accent);
  display: inline;
}

.glossary dt::after { content: " — "; color: var(--muted); }

.glossary dd {
  display: inline;
  margin: 0;
  color: var(--text);
}

.glossary dd::after {
  content: "";
  display: block;
  margin-bottom: 0.25rem;
}

.recap-list {
  list-style: none;
  padding: 0;
  margin: 0;
  display: flex;
  flex-direction: column;
  gap: 0.6rem;
}

.recap-list a {
  display: block;
  padding: 0.65rem 0.85rem;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  text-decoration: none;
  color: inherit;
}

.recap-list a:hover { border-color: var(--accent); }

.btn {
  background: var(--accent-dim);
  color: var(--text);
  border: none;
  border-radius: 6px;
  padding: 0.5rem 1rem;
  font-size: 0.9rem;
  cursor: pointer;
  font-family: var(--font);
}

.btn:hover { background: var(--accent); }

.partition-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 0.5rem;
  margin: 1rem 0;
  text-align: center;
  font-size: 0.85rem;
}

.partition-cell {
  padding: 0.75rem;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
}

@media (max-width: 600px) {
  .chart-wrap { height: 220px; }
}