:root {
  --bg: #0f1720;
  --panel: #182231;
  --border: #2a3648;
  --text: #e7edf5;
  --muted: #93a2b8;
  --accent: #5aa7ff;
}
@media (prefers-color-scheme: light) {
  :root {
    --bg: #f5f7fa;
    --panel: #ffffff;
    --border: #e1e6ed;
    --text: #1a2333;
    --muted: #5c6b80;
    --accent: #2064c8;
  }
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
}
header {
  padding: 2.5rem 1.5rem 2rem;
  text-align: center;
  border-bottom: 1px solid var(--border);
  position: relative;
}
header h1 {
  margin: 0 0 0.4rem;
  font-size: clamp(1.6rem, 4vw, 2.2rem);
}
header p {
  margin: 0;
  color: var(--muted);
}
a.back {
  display: inline-block;
  margin-bottom: 1rem;
  color: var(--muted);
  text-decoration: none;
  font-size: 0.9rem;
}
a.back:hover { color: var(--accent); }

/* menu (homepage) */
main.menu {
  max-width: 1000px;
  margin: 0 auto;
  padding: 2.5rem 1.5rem 4rem;
  display: grid;
  gap: 1.5rem;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
}
a.tile {
  display: block;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 1.75rem 1.5rem;
  text-decoration: none;
  color: var(--text);
  transition: border-color 0.15s ease;
}
a.tile:hover {
  border-color: var(--accent);
}
a.tile h2 {
  margin: 0 0 0.35rem;
  font-size: 1.2rem;
}
a.tile p {
  margin: 0;
  color: var(--muted);
  font-size: 0.85rem;
}

/* topic page */
main.topic {
  max-width: 700px;
  margin: 0 auto;
  padding: 2rem 1.5rem 4rem;
}
main.topic h1 {
  font-size: 1.6rem;
  margin: 0 0 0.25rem;
}
main.topic .intro {
  color: var(--muted);
  margin: 0 0 1.5rem;
  font-size: 0.9rem;
}
ul.links {
  list-style: none;
  margin: 0;
  padding: 0;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 12px;
  overflow: hidden;
}
ul.links li {
  border-top: 1px solid var(--border);
}
ul.links li:first-child {
  border-top: none;
}
ul.links a {
  display: block;
  padding: 0.85rem 1.1rem;
  color: var(--text);
  text-decoration: none;
}
ul.links a:hover {
  color: var(--accent);
}
ul.links a span.desc {
  display: block;
  color: var(--muted);
  font-size: 0.8rem;
  margin-top: 0.15rem;
}
.note {
  color: #d9a441;
  font-size: 0.78rem;
  margin-top: 0.15rem;
}
.empty {
  color: var(--muted);
  font-size: 0.9rem;
  font-style: italic;
  padding: 1rem 0;
}
footer {
  text-align: center;
  color: var(--muted);
  font-size: 0.8rem;
  padding: 2rem 1.5rem 3rem;
}