/* ── Reset & base ─────────────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; }

:root {
  --text:        #1a1a2e;
  --muted:       #555;
  --accent:      #2d5fa8;
  --accent-dark: #1a3e73;
  --bg:          #ffffff;
  --bg-alt:      #f6f8fa;
  --border:      #d1d9e0;
  --code-bg:     #f0f2f5;
  --code-border: #c8d0da;
  --max-width:   780px;
  --font-sans:   system-ui, -apple-system, "Segoe UI", Helvetica, Arial, sans-serif;
  --font-mono:   "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
}

html { font-size: 16px; scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: var(--font-sans);
  color: var(--text);
  background: var(--bg);
  line-height: 1.7;
}

/* ── Site header ──────────────────────────────────────────────────────────── */
header.site-header {
  background: var(--accent);
  color: #fff;
  padding: 0 2rem;
}

header.site-header .inner {
  max-width: var(--max-width);
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 56px;
}

header.site-header a.site-title {
  color: #fff;
  text-decoration: none;
  font-weight: 700;
  font-size: 1.05rem;
  letter-spacing: 0.01em;
}

header.site-header nav a {
  color: rgba(255,255,255,0.85);
  text-decoration: none;
  margin-left: 1.4rem;
  font-size: 0.9rem;
  transition: color 0.15s;
}
header.site-header nav a:hover { color: #fff; }

/* ── Lecture breadcrumb strip ─────────────────────────────────────────────── */
.lecture-nav {
  background: var(--bg-alt);
  border-bottom: 1px solid var(--border);
  padding: 0 2rem;
}
.lecture-nav .inner {
  max-width: var(--max-width);
  margin: 0 auto;
  display: flex;
  gap: 0;
}
.lecture-nav a {
  display: inline-block;
  padding: 0.55rem 1rem;
  font-size: 0.85rem;
  color: var(--muted);
  text-decoration: none;
  border-bottom: 3px solid transparent;
  transition: color 0.15s, border-color 0.15s;
}
.lecture-nav a:hover { color: var(--accent); }
.lecture-nav a.active {
  color: var(--accent);
  border-bottom-color: var(--accent);
  font-weight: 600;
}

/* ── Main content ─────────────────────────────────────────────────────────── */
main {
  max-width: var(--max-width);
  margin: 2.5rem auto 4rem;
  padding: 0 1.5rem;
}

/* ── Hero (index page only) ───────────────────────────────────────────────── */
.hero {
  background: linear-gradient(135deg, #1a3e73 0%, #2d5fa8 100%);
  color: #fff;
  padding: 3.5rem 2rem;
  text-align: center;
}
.hero h1 {
  margin: 0 0 0.4rem;
  font-size: 2rem;
  font-weight: 800;
  letter-spacing: -0.02em;
}
.hero .subtitle {
  margin: 0 auto 1.8rem;
  max-width: 520px;
  font-size: 1.05rem;
  color: rgba(255,255,255,0.85);
}
.hero .btn-row { display: flex; gap: 1rem; justify-content: center; flex-wrap: wrap; }
.btn {
  display: inline-block;
  padding: 0.6rem 1.4rem;
  border-radius: 5px;
  font-size: 0.95rem;
  font-weight: 600;
  text-decoration: none;
  transition: opacity 0.15s;
}
.btn:hover { opacity: 0.85; }
.btn-primary { background: #fff; color: var(--accent-dark); }
.btn-secondary { background: transparent; color: #fff; border: 2px solid rgba(255,255,255,0.6); }

/* ── Lecture card grid ────────────────────────────────────────────────────── */
.lecture-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(340px, 1fr));
  gap: 1.2rem;
  margin: 2.5rem auto;
  max-width: var(--max-width);
  padding: 0 1.5rem;
}
.lecture-card {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.4rem 1.6rem;
  text-decoration: none;
  color: inherit;
  transition: box-shadow 0.15s, border-color 0.15s;
  display: block;
}
.lecture-card:hover {
  border-color: var(--accent);
  box-shadow: 0 2px 12px rgba(45,95,168,0.12);
}
.lecture-card .lecture-num {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--accent);
  margin-bottom: 0.4rem;
}
.lecture-card h3 {
  margin: 0 0 0.5rem;
  font-size: 1.05rem;
}
.lecture-card p {
  margin: 0;
  font-size: 0.875rem;
  color: var(--muted);
  line-height: 1.5;
}

/* ── Info section (index) ─────────────────────────────────────────────────── */
.info-section {
  max-width: var(--max-width);
  margin: 0 auto 3rem;
  padding: 0 1.5rem;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 2rem;
}
@media (max-width: 600px) { .info-section { grid-template-columns: 1fr; } }
.info-section h2 { margin-top: 0; font-size: 1.15rem; }

/* ── Typography ───────────────────────────────────────────────────────────── */
h1, h2, h3, h4 {
  line-height: 1.3;
  font-weight: 700;
  margin-top: 2rem;
  margin-bottom: 0.6rem;
}
h1 { font-size: 1.9rem; margin-top: 0; }
h2 { font-size: 1.35rem; border-bottom: 1px solid var(--border); padding-bottom: 0.3rem; }
h3 { font-size: 1.1rem; }
h4 { font-size: 1rem; font-weight: 600; }

p { margin: 0.8rem 0 1rem; }

a { color: var(--accent); text-decoration: underline; text-underline-offset: 2px; }
a:hover { color: var(--accent-dark); }

/* ── Code ─────────────────────────────────────────────────────────────────── */
code {
  font-family: var(--font-mono);
  font-size: 0.875em;
  background: var(--code-bg);
  border: 1px solid var(--code-border);
  border-radius: 3px;
  padding: 0.1em 0.35em;
}

pre {
  background: var(--code-bg);
  border: 1px solid var(--code-border);
  border-radius: 6px;
  padding: 1rem 1.2rem;
  overflow-x: auto;
  font-size: 0.875rem;
  line-height: 1.55;
}
pre code {
  background: none;
  border: none;
  padding: 0;
  font-size: inherit;
}

/* Pandoc syntax-highlighting overrides */
div.sourceCode { margin: 1rem 0; }
div.sourceCode pre { margin: 0; }

/* ── Tables ───────────────────────────────────────────────────────────────── */
table {
  border-collapse: collapse;
  width: 100%;
  margin: 1.2rem 0;
  font-size: 0.9rem;
}
th {
  background: var(--bg-alt);
  border: 1px solid var(--border);
  padding: 0.5rem 0.85rem;
  text-align: left;
  font-weight: 600;
}
td {
  border: 1px solid var(--border);
  padding: 0.45rem 0.85rem;
}
tr:nth-child(even) td { background: var(--bg-alt); }

/* ── Blockquote ───────────────────────────────────────────────────────────── */
blockquote {
  border-left: 4px solid var(--accent);
  margin: 1rem 0;
  padding: 0.4rem 1rem;
  color: var(--muted);
  background: var(--bg-alt);
  border-radius: 0 4px 4px 0;
}
blockquote p { margin: 0.3rem 0; }

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

/* ── Footer ───────────────────────────────────────────────────────────────── */
footer {
  border-top: 1px solid var(--border);
  padding: 1.2rem 2rem;
  text-align: center;
  font-size: 0.82rem;
  color: var(--muted);
}
footer a { color: var(--muted); }

/* ── Responsive ───────────────────────────────────────────────────────────── */
@media (max-width: 480px) {
  header.site-header .inner { height: auto; padding: 0.6rem 0; flex-direction: column; align-items: flex-start; gap: 0.3rem; }
  header.site-header nav { display: flex; flex-wrap: wrap; gap: 0.4rem; }
  header.site-header nav a { margin-left: 0; }
  .hero { padding: 2rem 1rem; }
  .hero h1 { font-size: 1.5rem; }
}