:root {
  color-scheme: light dark;
  --bg: #0f1419;
  --bg-card: #1a2332;
  --text: #e7ecf3;
  --muted: #9aa8bc;
  --accent: #5b9fd4;
  --accent2: #7c6cf0;
  --border: rgba(255, 255, 255, 0.08);
  --radius: 12px;
  --font: system-ui, -apple-system, "Segoe UI", Roboto, Ubuntu, sans-serif;
}

@media (prefers-color-scheme: light) {
  :root {
    --bg: #f4f6fa;
    --bg-card: #fff;
    --text: #1a2332;
    --muted: #5c6b80;
    --border: rgba(0, 0, 0, 0.08);
  }
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
  min-height: 100vh;
}

.inner {
  width: min(920px, 100% - 2rem);
  margin-inline: auto;
}

.site-header {
  padding: 2.5rem 0 2rem;
  border-bottom: 1px solid var(--border);
  background: linear-gradient(165deg, rgba(91, 159, 212, 0.12), transparent 55%);
}

.site-header h1 {
  margin: 0 0 0.5rem;
  font-size: clamp(1.5rem, 4vw, 2rem);
  font-weight: 650;
  letter-spacing: -0.02em;
}

.tagline {
  margin: 0 0 1.25rem;
  color: var(--muted);
  max-width: 52ch;
}

.toc {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
}

.nav-link {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 2.5rem;
  padding: 0.35rem 0.6rem;
  border-radius: 8px;
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--accent);
  text-decoration: none;
  border: 1px solid var(--border);
  background: var(--bg-card);
  transition: transform 0.12s ease, border-color 0.12s ease;
}

.nav-link:hover {
  transform: translateY(-1px);
  border-color: var(--accent);
}

main {
  padding: 2rem 0 3rem;
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

.module {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.25rem 1.35rem;
  scroll-margin-top: 1rem;
}

.module-head {
  display: flex;
  gap: 1rem;
  align-items: flex-start;
  margin-bottom: 1rem;
}

.module-num {
  flex-shrink: 0;
  font-size: 0.75rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  color: var(--muted);
  background: rgba(91, 159, 212, 0.15);
  color: var(--accent);
  padding: 0.35rem 0.55rem;
  border-radius: 6px;
}

.module h2 {
  margin: 0 0 0.35rem;
  font-size: 1.15rem;
  font-weight: 650;
}

.module-blurb {
  margin: 0;
  color: var(--muted);
  font-size: 0.95rem;
}

.preview-details {
  margin: 0 0 1rem;
  border: 1px solid var(--border);
  border-radius: 10px;
  overflow: hidden;
  background: rgba(0, 0, 0, 0.15);
}

@media (prefers-color-scheme: light) {
  .preview-details {
    background: rgba(0, 0, 0, 0.03);
  }
}

.preview-summary {
  cursor: pointer;
  padding: 0.65rem 0.85rem;
  font-size: 0.88rem;
  font-weight: 600;
  color: var(--muted);
  list-style: none;
  user-select: none;
}

.preview-summary::-webkit-details-marker {
  display: none;
}

.preview-summary::before {
  content: "▸ ";
  display: inline-block;
  transition: transform 0.15s ease;
}

details[open] > .preview-summary::before {
  transform: rotate(90deg);
}

.preview-shell {
  position: relative;
  width: 100%;
  min-height: 420px;
  height: min(62vh, 640px);
  border-top: 1px solid var(--border);
}

.preview-iframe {
  display: block;
  width: 100%;
  height: 100%;
  min-height: 420px;
  border: 0;
  background: var(--bg);
}

.preview-meta {
  margin: 0;
  padding: 0.5rem 0.85rem 0.75rem;
  font-size: 0.82rem;
  color: var(--muted);
}

.preview-meta a {
  color: var(--accent);
}

.preview-fallback {
  margin: 0 0 1rem;
  padding: 0.75rem 0.9rem;
  border-radius: 10px;
  border: 1px dashed var(--border);
  font-size: 0.88rem;
  color: var(--muted);
}

.preview-fallback code {
  font-size: 0.85em;
}

.module-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.6rem;
}

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  padding: 0.55rem 1rem;
  border-radius: 8px;
  font-size: 0.9rem;
  font-weight: 600;
  text-decoration: none;
  border: 1px solid transparent;
  cursor: pointer;
  transition: opacity 0.12s ease, transform 0.12s ease;
}

.btn:hover {
  opacity: 0.92;
  transform: translateY(-1px);
}

.btn-ipynb {
  background: linear-gradient(135deg, #f37626, #e35a0c);
  color: #fff;
}

.btn-pdf {
  background: linear-gradient(135deg, var(--accent2), #5b4dc4);
  color: #fff;
}

.btn-viewer {
  background: transparent;
  color: var(--accent);
  border: 1px solid var(--accent);
}

.btn-viewer:hover {
  background: rgba(91, 159, 212, 0.12);
}

.btn-disabled {
  background: var(--border);
  color: var(--muted);
  cursor: not-allowed;
  font-weight: 500;
}

.btn-disabled:hover {
  transform: none;
  opacity: 1;
}

.site-footer {
  padding: 1.5rem 0 2.5rem;
  border-top: 1px solid var(--border);
  font-size: 0.85rem;
  color: var(--muted);
}

.site-footer code {
  font-size: 0.9em;
  padding: 0.1em 0.35em;
  border-radius: 4px;
  background: var(--bg-card);
}