:root {
  --bg: #0f1115;
  --panel: #161a22;
  --panel-2: #1d222d;
  --text: #e6e8eb;
  --muted: #98a2b3;
  --accent: #3ea6ff;
  --accent-2: #5acc80;
  --warn: #ffb454;
  --err: #ff6b6b;
  --border: #262d3a;
  --code: #c8d3e1;
  --mono: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
}

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
body {
  background: var(--bg);
  color: var(--text);
  font: 16px/1.55 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
main { flex: 1; max-width: 960px; width: 100%; margin: 0 auto; padding: 24px 20px 64px; }
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

.site-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 20px;
  border-bottom: 1px solid var(--border);
  background: var(--panel);
}
.brand { font-weight: 700; font-size: 1.1rem; color: var(--text); }
.site-header nav { display: flex; gap: 18px; }
.site-footer {
  padding: 16px 20px;
  border-top: 1px solid var(--border);
  color: var(--muted);
  font-size: 0.85rem;
  text-align: center;
}

h1, h2, h3 { line-height: 1.25; }
h1 { font-size: 1.75rem; margin: 0 0 6px; }
.subtitle { color: var(--muted); margin: 0 0 24px; }

.filters {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 12px;
  padding: 12px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 8px;
  margin-bottom: 18px;
}
.filters label { font-size: 0.85rem; color: var(--muted); display: flex; gap: 6px; align-items: center; }
.filters select, .filters input[type="search"] {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 6px 8px;
  font: inherit;
}

.challenge-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 12px;
}
.card {
  display: block;
  padding: 14px 16px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 8px;
  color: var(--text);
}
.card:hover { border-color: var(--accent); text-decoration: none; }
.card h3 { margin: 0 0 6px; font-size: 1rem; }
.card .meta { display: flex; gap: 8px; flex-wrap: wrap; font-size: 0.78rem; color: var(--muted); }
.card .meta .tag {
  padding: 2px 8px;
  background: var(--panel-2);
  border: 1px solid var(--border);
  border-radius: 999px;
}
.card .meta .tag.solved { color: var(--accent-2); border-color: var(--accent-2); }
.diff { letter-spacing: 1px; color: var(--warn); }

.problem {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 18px 20px;
  margin-bottom: 16px;
}
.problem .meta-row { display: flex; gap: 10px; flex-wrap: wrap; margin: 4px 0 16px; color: var(--muted); font-size: 0.85rem; }
.problem .meta-row .tag {
  padding: 2px 8px;
  background: var(--panel-2);
  border: 1px solid var(--border);
  border-radius: 999px;
}

table.input-table {
  border-collapse: collapse;
  margin: 8px 0 12px;
  font-family: var(--mono);
  font-size: 0.92rem;
}
table.input-table th, table.input-table td {
  border: 1px solid var(--border);
  padding: 4px 10px;
  background: var(--panel-2);
}
table.input-table th { background: #11141b; color: var(--muted); font-weight: 500; }
.range-hint { color: var(--muted); font-size: 0.85rem; margin-top: -4px; }

.checker {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 18px 20px;
  margin-bottom: 16px;
}
.checker label { display: block; color: var(--muted); font-size: 0.85rem; margin-bottom: 6px; }
.checker textarea, .checker input[type="text"] {
  width: 100%;
  background: #0b0d12;
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 10px;
  font: 0.95rem var(--mono);
  resize: vertical;
}
.checker textarea { min-height: 80px; }
.checker .row { display: flex; gap: 8px; align-items: center; margin-top: 10px; flex-wrap: wrap; }
.checker button {
  background: var(--accent);
  color: #061522;
  border: 0;
  border-radius: 6px;
  padding: 8px 14px;
  font: 600 0.9rem system-ui, sans-serif;
  cursor: pointer;
}
.checker button.secondary {
  background: transparent;
  color: var(--text);
  border: 1px solid var(--border);
}
.checker .feedback { font-family: var(--mono); font-size: 0.9rem; }
.checker .feedback.ok { color: var(--accent-2); }
.checker .feedback.no { color: var(--err); }

details.disclosure {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 8px 16px;
  margin-bottom: 12px;
}
details.disclosure summary {
  cursor: pointer;
  padding: 6px 0;
  user-select: none;
  font-weight: 600;
}
details.disclosure summary::-webkit-details-marker { color: var(--muted); }
details.disclosure pre {
  background: #0b0d12;
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 12px;
  overflow-x: auto;
  font: 0.9rem var(--mono);
  color: var(--code);
  white-space: pre;
}

.prose p { margin: 0 0 10px; }
.prose code {
  font: 0.9em var(--mono);
  background: var(--panel-2);
  padding: 1px 5px;
  border-radius: 4px;
}
.prose pre {
  background: #0b0d12;
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 12px;
  overflow-x: auto;
  font: 0.9rem var(--mono);
}

@media (max-width: 600px) {
  main { padding: 16px 12px 40px; }
  .site-header { padding: 12px 14px; }
  h1 { font-size: 1.4rem; }
}