:root {
  --bg: #0d1117;
  --bg-elev: #161b22;
  --bg-elev-2: #1c2230;
  --border: #2a3038;
  --text: #e6edf3;
  --text-dim: #9aa4b2;
  --accent: #4f8cff;
  --accent-2: #22d3a6;
  --easy: #22c55e;
  --medium: #eab308;
  --hard: #ef4444;
  --radius: 12px;
  --maxw: 900px;
  font-synthesis: none;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.6;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

/* ---------- Top bar ---------- */
.topbar {
  position: sticky;
  top: 0;
  z-index: 50;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.75rem 1.25rem;
  background: rgba(13, 17, 23, 0.85);
  backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--border);
}
.brand {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  font-weight: 700;
  font-size: 1.1rem;
  color: var(--text);
}
.brand:hover { text-decoration: none; }
.logo {
  display: inline-grid;
  place-items: center;
  width: 32px; height: 32px;
  background: linear-gradient(135deg, var(--accent), var(--accent-2));
  border-radius: 8px;
  color: #06121f;
  font-weight: 800;
  font-size: 0.85rem;
}
.topnav { display: flex; align-items: center; gap: 1.25rem; }
.topnav a { color: var(--text-dim); font-weight: 500; }
.topnav a:hover, .topnav a.active { color: var(--text); text-decoration: none; }
.progress-badge {
  background: var(--bg-elev-2);
  border: 1px solid var(--border);
  padding: 0.25rem 0.7rem;
  border-radius: 999px;
  font-size: 0.8rem;
  color: var(--accent-2);
  font-weight: 600;
}

/* ---------- Layout ---------- */
.app {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 2rem 1.25rem 4rem;
  min-height: calc(100vh - 130px);
}
.footer {
  text-align: center;
  color: var(--text-dim);
  font-size: 0.8rem;
  padding: 1.5rem;
  border-top: 1px solid var(--border);
}

/* ---------- Home / hero ---------- */
.hero { text-align: center; margin-bottom: 2.5rem; }
.hero h1 {
  font-size: 2.4rem;
  margin: 0 0 0.5rem;
  background: linear-gradient(135deg, var(--text), var(--accent));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}
.hero p { color: var(--text-dim); font-size: 1.05rem; max-width: 620px; margin: 0 auto; }

.section-title {
  font-size: 1.3rem;
  margin: 2rem 0 1rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

/* ---------- Roadmap graph ---------- */
.roadmap-wrap {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.25rem;
  overflow-x: auto;
  margin-bottom: 2rem;
}
.mermaid { text-align: center; }

/* ---------- Topic cards ---------- */
.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 1rem;
}
.topic-card {
  display: block;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.1rem 1.2rem;
  transition: border-color .15s, transform .15s;
}
.topic-card:hover { border-color: var(--accent); transform: translateY(-2px); text-decoration: none; }
.topic-card h3 { margin: 0 0 0.35rem; color: var(--text); font-size: 1.05rem; }
.topic-card p { margin: 0 0 0.75rem; color: var(--text-dim); font-size: 0.85rem; }
.topic-meta { display: flex; align-items: center; justify-content: space-between; font-size: 0.8rem; color: var(--text-dim); }

.progress-bar {
  height: 6px;
  background: var(--bg-elev-2);
  border-radius: 999px;
  overflow: hidden;
  margin-top: 0.6rem;
}
.progress-bar > span {
  display: block;
  height: 100%;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
}

/* ---------- Problem list ---------- */
.breadcrumb { color: var(--text-dim); font-size: 0.85rem; margin-bottom: 1rem; }
.problem-table { width: 100%; border-collapse: collapse; }
.problem-table th, .problem-table td {
  text-align: left;
  padding: 0.7rem 0.6rem;
  border-bottom: 1px solid var(--border);
  font-size: 0.92rem;
}
.problem-table th { color: var(--text-dim); font-weight: 600; font-size: 0.8rem; text-transform: uppercase; letter-spacing: .04em; }
.problem-table tr:hover td { background: var(--bg-elev); }
.difficulty { font-weight: 700; font-size: 0.8rem; }
.difficulty.Easy { color: var(--easy); }
.difficulty.Medium { color: var(--medium); }
.difficulty.Hard { color: var(--hard); }
.check {
  width: 20px; height: 20px;
  border-radius: 6px;
  border: 1px solid var(--border);
  background: var(--bg-elev-2);
  cursor: pointer;
  display: inline-grid;
  place-items: center;
  color: transparent;
  font-size: 0.75rem;
}
.check.done { background: var(--accent-2); border-color: var(--accent-2); color: #06121f; }

/* ---------- Problem lesson page ---------- */
.lesson h1 { font-size: 1.9rem; margin: 0.2rem 0 0.4rem; }
.lesson .tags { display: flex; gap: 0.5rem; flex-wrap: wrap; margin-bottom: 1.5rem; align-items: center; }
.tag {
  background: var(--bg-elev-2);
  border: 1px solid var(--border);
  padding: 0.2rem 0.6rem;
  border-radius: 999px;
  font-size: 0.75rem;
  color: var(--text-dim);
}
.lesson-content { }
.lesson-content h2 {
  font-size: 1.35rem;
  margin: 2.2rem 0 0.8rem;
  padding-bottom: 0.4rem;
  border-bottom: 1px solid var(--border);
}
.lesson-content h3 { font-size: 1.1rem; margin: 1.5rem 0 0.5rem; color: var(--accent-2); }
.lesson-content p, .lesson-content li { color: var(--text); }
.lesson-content code {
  background: var(--bg-elev-2);
  padding: 0.12rem 0.4rem;
  border-radius: 5px;
  font-size: 0.88em;
  font-family: "SFMono-Regular", Consolas, "Liberation Mono", monospace;
}
.lesson-content pre {
  background: #0b0f16 !important;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1rem 1.1rem;
  overflow-x: auto;
}
.lesson-content pre code { background: none; padding: 0; font-size: 0.86rem; line-height: 1.5; }
.lesson-content blockquote {
  border-left: 3px solid var(--accent);
  margin: 1rem 0;
  padding: 0.4rem 1rem;
  background: var(--bg-elev);
  border-radius: 0 8px 8px 0;
  color: var(--text-dim);
}
.lesson-content table { border-collapse: collapse; width: 100%; margin: 1rem 0; }
.lesson-content th, .lesson-content td { border: 1px solid var(--border); padding: 0.5rem 0.7rem; font-size: 0.88rem; }
.lesson-content th { background: var(--bg-elev-2); }
.callout {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-left: 3px solid var(--accent-2);
  border-radius: 0 8px 8px 0;
  padding: 0.8rem 1rem;
  margin: 1.2rem 0;
}
.callout strong { color: var(--accent-2); }

.lesson-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
  gap: 1rem;
}
.lesson-nav a {
  flex: 1;
  padding: 0.9rem 1rem;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
}
.lesson-nav a:hover { border-color: var(--accent); text-decoration: none; }
.lesson-nav a.next { text-align: right; }
.lesson-nav small { color: var(--text-dim); display: block; }

.complete-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  background: var(--accent-2);
  color: #06121f;
  border: none;
  padding: 0.6rem 1.2rem;
  border-radius: 8px;
  font-weight: 700;
  cursor: pointer;
  font-size: 0.9rem;
}
.complete-btn.done { background: var(--bg-elev-2); color: var(--text-dim); }

.mermaid-block {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1rem;
  margin: 1.2rem 0;
  overflow-x: auto;
}

.empty-state { text-align: center; color: var(--text-dim); padding: 3rem 1rem; }

/* ---------- Concept guide banner ---------- */
.guide-banner {
  display: flex;
  align-items: center;
  gap: 1rem;
  background: linear-gradient(135deg, rgba(79,140,255,0.12), rgba(34,211,166,0.12));
  border: 1px solid var(--accent);
  border-radius: var(--radius);
  padding: 1rem 1.2rem;
  margin: 1.25rem 0 0.5rem;
  color: var(--text);
}
.guide-banner:hover { text-decoration: none; border-color: var(--accent-2); transform: translateY(-2px); transition: transform .15s, border-color .15s; }
.guide-banner .guide-icon { font-size: 1.6rem; }
.guide-banner strong { display: block; font-size: 1.05rem; }
.guide-banner small { color: var(--text-dim); }
.guide-banner .guide-arrow { margin-left: auto; font-size: 1.3rem; color: var(--accent); }

/* ---------- Extra learning tracks ---------- */
.track-section { margin-top: 2.5rem; }
.track-count {
  font-size: 0.8rem;
  color: var(--accent-2);
  background: var(--bg-elev-2);
  border: 1px solid var(--border);
  border-radius: 999px;
  padding: 0.15rem 0.6rem;
  margin-left: 0.5rem;
  font-weight: 600;
}
.lesson-num {
  display: inline-grid;
  place-items: center;
  width: 22px; height: 22px;
  background: var(--bg-elev-2);
  border: 1px solid var(--border);
  border-radius: 6px;
  font-size: 0.75rem;
  color: var(--accent);
  margin-right: 0.35rem;
  vertical-align: middle;
}