:root {
  --bg: #15161a;
  --surface: #1e2025;
  --surface-hover: #262933;
  --text: #e9e9ec;
  --muted: #9a9ca3;
  --accent: #ff5f3d;
  --easy: #3ddc84;
  --medium: #ffb84d;
  --hard: #ff5f5f;
  --border: #30323a;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: "Helvetica Neue", Arial, sans-serif;
  line-height: 1.55;
}

main {
  max-width: 840px;
  margin: 0 auto;
  padding: 0 24px 64px;
}

.site-header {
  padding: 48px 24px 32px;
  max-width: 840px;
  margin: 0 auto;
}

.brand {
  color: var(--text);
  text-decoration: none;
  font-size: 2rem;
  font-weight: 800;
  letter-spacing: -0.02em;
}

.tagline {
  color: var(--accent);
  font-weight: 600;
  margin: 8px 0 0;
}

.intro p {
  color: var(--muted);
  font-size: 1.05rem;
}

.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 16px;
  margin-top: 24px;
}

.topic-card {
  display: block;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 20px;
  text-decoration: none;
  color: var(--text);
  transition: background 0.15s ease, border-color 0.15s ease;
}

.topic-card:hover {
  background: var(--surface-hover);
  border-color: var(--accent);
}

.topic-card h2 {
  margin: 0 0 8px;
  font-size: 1.15rem;
}

.topic-card p {
  margin: 0 0 12px;
  color: var(--muted);
  font-size: 0.92rem;
}

.rep-count {
  font-size: 0.78rem;
  color: var(--accent);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.03em;
}

.back-link {
  display: inline-block;
  margin: 32px 0 8px;
  color: var(--muted);
  text-decoration: none;
  font-size: 0.9rem;
}

.back-link:hover {
  color: var(--accent);
}

.topic-header h1 {
  margin: 8px 0 4px;
}

.topic-header .summary {
  color: var(--muted);
}

.section-label {
  text-transform: uppercase;
  letter-spacing: 0.06em;
  font-size: 0.85rem;
  color: var(--accent);
  border-bottom: 1px solid var(--border);
  padding-bottom: 8px;
  margin-top: 40px;
}

.problem {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 20px 24px;
  margin-top: 20px;
}

.problem h3 {
  margin: 0 0 8px;
}

.problem h3 a {
  color: var(--text);
  text-decoration: none;
}

.problem h3 a:hover {
  color: var(--accent);
}

.problem-statement {
  color: var(--muted);
  margin: 0 0 16px;
}

.solution {
  background: #0f1013;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 16px;
  overflow-x: auto;
  font-size: 0.88rem;
}

.solution code {
  font-family: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
}

.explanation {
  margin: 16px 0 0;
  color: var(--muted);
}

.level-up p {
  color: var(--muted);
}

.harder-list {
  list-style: none;
  padding: 0;
  margin: 16px 0 0;
}

.harder-list li {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px 16px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  margin-bottom: 8px;
}

.harder-list a {
  color: var(--text);
  text-decoration: none;
}

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

.difficulty {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  padding: 2px 8px;
  border-radius: 999px;
}

.difficulty-medium {
  color: var(--medium);
  background: rgba(255, 184, 77, 0.12);
}

.difficulty-hard {
  color: var(--hard);
  background: rgba(255, 95, 95, 0.12);
}

.site-footer {
  text-align: center;
  color: var(--muted);
  font-size: 0.85rem;
  padding: 24px;
}