:root {
  --bg: #0e1117;
  --bg-soft: #161b22;
  --bg-elev: #1c2129;
  --bg-code: #0a0d12;
  --border: #2a3038;
  --text: #e6edf3;
  --text-dim: #9aa4b2;
  --accent: #58a6ff;
  --accent-2: #7ee787;
  --accent-3: #ffa657;
  --accent-4: #f778ba;
  --danger: #f85149;
  --radius: 10px;
  --mono: "SFMono-Regular", ui-monospace, Menlo, Consolas, "Liberation Mono", monospace;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.65;
  font-size: 16px;
  -webkit-tap-highlight-color: transparent;
}

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

/* ===== Layout ===== */
.app {
  display: grid;
  grid-template-columns: 280px 1fr;
  min-height: 100vh;
}

.sidebar {
  background: var(--bg-soft);
  border-right: 1px solid var(--border);
  padding: 20px 16px;
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.brand {
  font-size: 20px;
  font-weight: 700;
  letter-spacing: 0.3px;
  margin-bottom: 4px;
}
.brand .angle { color: var(--accent-3); }
.brand-sub {
  color: var(--text-dim);
  font-size: 13px;
  margin-bottom: 18px;
}

/* Mobile nav toggle (hidden on desktop) */
.nav-toggle {
  display: none;
}

.nav { list-style: none; padding: 0; margin: 0; }
.nav li { margin: 2px 0; }
.nav a {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 12px;
  border-radius: 8px;
  color: var(--text-dim);
  font-size: 14px;
  transition: background .15s, color .15s;
}
.nav a.active { background: var(--bg-elev); color: var(--text); }
@media (hover: hover) {
  .nav a:hover { background: var(--bg-elev); color: var(--text); text-decoration: none; }
}
.nav a .num {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 22px; height: 22px;
  background: var(--bg-code);
  border: 1px solid var(--border);
  border-radius: 6px;
  font-size: 12px;
  font-family: var(--mono);
  color: var(--accent);
  flex-shrink: 0;
}
.nav a.active .num { background: var(--accent); color: #0a0d12; border-color: var(--accent); font-weight: 700; }

.nav-section {
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 1px;
  color: var(--text-dim);
  margin: 18px 0 6px 12px;
}

.main { padding: 32px 40px; max-width: 960px; }

/* ===== Hero / Welcome ===== */
.hero { margin-bottom: 32px; }
.hero h1 {
  font-size: 36px;
  margin: 0 0 8px;
  line-height: 1.2;
}
.hero h1 .angle { color: var(--accent-3); }
.hero p.lead { color: var(--text-dim); font-size: 17px; max-width: 720px; }

.how-to {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: 14px;
  margin: 24px 0 8px;
}
.how-card {
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 16px;
}
.how-card .step {
  color: var(--accent);
  font-weight: 700;
  font-size: 14px;
  text-transform: uppercase;
  letter-spacing: 1px;
  margin-bottom: 6px;
}
.how-card p { margin: 0; font-size: 14px; color: var(--text-dim); }

/* ===== Lesson content ===== */
.lesson { display: none; }
.lesson.active { display: block; }

.lesson h2 {
  font-size: 28px;
  margin: 0 0 6px;
}
.lesson .lesson-tag {
  display: inline-block;
  background: var(--accent);
  color: #0a0d12;
  font-size: 12px;
  font-weight: 700;
  padding: 2px 10px;
  border-radius: 20px;
  margin-bottom: 14px;
}
.lesson .explanation p { margin: 10px 0; }
.lesson .explanation ul, .lesson .explanation ol { padding-left: 22px; }
.lesson .explanation li { margin: 5px 0; }

.lesson .explanation .callout {
  background: var(--bg-soft);
  border-left: 3px solid var(--accent-3);
  padding: 10px 14px;
  border-radius: 6px;
  margin: 14px 0;
  font-size: 14px;
}

/* Code block */
.codeblock { position: relative; margin: 18px 0; }
.codeblock .code-label {
  font-size: 12px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: 1px;
  margin-bottom: 6px;
}
.codeblock pre {
  background: var(--bg-code);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 16px;
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
  margin: 0;
}
.codeblock code {
  font-family: var(--mono);
  font-size: 13.5px;
  line-height: 1.6;
  color: #c9d1d9;
}
.codeblock .tok-tag { color: #7ee787; }
.codeblock .tok-attr { color: #79c0ff; }
.codeblock .tok-str { color: #ffa657; }
.codeblock .tok-comment { color: #8b949e; font-style: italic; }

/* Live exercise */
.exercise {
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px;
  margin-top: 22px;
}
.exercise h3 {
  margin: 0 0 6px;
  font-size: 18px;
}
.exercise .ex-hint { color: var(--text-dim); font-size: 14px; margin: 0 0 14px; }
.exercise textarea {
  width: 100%;
  min-height: 180px;
  background: var(--bg-code);
  color: #c9d1d9;
  font-family: var(--mono);
  font-size: 13.5px;
  line-height: 1.6;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 14px;
  resize: vertical;
  outline: none;
}
.exercise textarea:focus { border-color: var(--accent); }
.ex-buttons { display: flex; gap: 10px; margin: 12px 0; flex-wrap: wrap; }

.btn {
  background: var(--accent);
  color: #0a0d12;
  border: none;
  padding: 9px 18px;
  border-radius: 8px;
  font-size: 14px;
  font-weight: 600;
  cursor: pointer;
  transition: filter .15s;
}
.btn.secondary { background: var(--bg-elev); color: var(--text); border: 1px solid var(--border); }
@media (hover: hover) {
  .btn:hover { filter: brightness(1.1); }
  .btn.secondary:hover { border-color: var(--accent); }
}

.preview {
  background: #ffffff;
  border: 1px solid var(--border);
  border-radius: 8px;
  margin-top: 4px;
}
.preview-label {
  font-size: 12px;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: 1px;
  margin: 14px 0 6px;
}
.preview iframe {
  width: 100%;
  min-height: 240px;
  border: none;
  background: #fff;
  border-radius: 8px;
  display: block;
}

/* ===== Cheat sheet ===== */
.cheatsheet h2 { font-size: 28px; }
.table-scroll {
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
  border-radius: 8px;
}
.cheatsheet table {
  width: 100%;
  border-collapse: collapse;
  margin: 16px 0;
  font-size: 14px;
}
.cheatsheet th, .cheatsheet td {
  text-align: left;
  padding: 8px 12px;
  border-bottom: 1px solid var(--border);
}
.cheatsheet th {
  background: var(--bg-soft);
  color: var(--text-dim);
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 1px;
}
.cheatsheet code {
  font-family: var(--mono);
  background: var(--bg-code);
  border: 1px solid var(--border);
  padding: 2px 7px;
  border-radius: 5px;
  font-size: 13px;
  color: #7ee787;
  white-space: nowrap;
}

/* ===== Quiz ===== */
.quiz h2 { font-size: 28px; }
.quiz .question {
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px;
  margin: 16px 0;
}
.quiz .q-text { font-weight: 600; margin-bottom: 12px; }
.quiz .option {
  display: block;
  padding: 10px 14px;
  margin: 6px 0;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: 8px;
  cursor: pointer;
  font-size: 15px;
  transition: border-color .15s, background .15s;
}
@media (hover: hover) {
  .quiz .option:hover { border-color: var(--accent); }
}
.quiz .option:active { border-color: var(--accent); }
.quiz .option.correct { border-color: var(--accent-2); background: rgba(126,231,135,0.12); }
.quiz .option.wrong { border-color: var(--danger); background: rgba(248,81,73,0.12); }
.quiz .option.disabled { pointer-events: none; }
.quiz .feedback {
  margin-top: 10px;
  font-size: 14px;
  padding: 10px 14px;
  border-radius: 8px;
  display: none;
}
.quiz .feedback.show { display: block; }
.quiz .feedback.correct { background: rgba(126,231,135,0.12); border: 1px solid var(--accent-2); }
.quiz .feedback.wrong { background: rgba(248,81,73,0.12); border: 1px solid var(--danger); }

.quiz-score {
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px;
  text-align: center;
  margin-top: 20px;
  display: none;
}
.quiz-score.show { display: block; }
.quiz-score .score-num { font-size: 40px; font-weight: 700; color: var(--accent); }

/* ===== Footer ===== */
footer {
  margin-top: 48px;
  padding-top: 20px;
  border-top: 1px solid var(--border);
  color: var(--text-dim);
  font-size: 13px;
}

/* ===== Responsive ===== */
@media (max-width: 860px) {
  .app { grid-template-columns: 1fr; }

  .sidebar {
    position: sticky;
    top: 0;
    z-index: 20;
    height: auto;
    max-height: none;
    overflow-y: visible;
    border-right: none;
    border-bottom: 1px solid var(--border);
    padding: 12px 14px;
  }
  .brand { font-size: 18px; padding-right: 110px; }
  .brand-sub { margin-bottom: 4px; }

  .nav-toggle {
    display: flex;
    align-items: center;
    justify-content: center;
    position: absolute;
    top: 10px;
    right: 14px;
    min-height: 44px;
    min-width: 44px;
    padding: 0 14px;
    background: var(--bg-elev);
    color: var(--text);
    border: 1px solid var(--border);
    border-radius: 8px;
    font-size: 15px;
    font-weight: 600;
    cursor: pointer;
    font-family: inherit;
  }
  .nav-toggle:active { border-color: var(--accent); }

  .sidebar .nav { display: none; }
  .sidebar.open .nav { display: block; }
  .nav a { min-height: 44px; font-size: 16px; padding: 10px 12px; }
  .nav-section { margin: 16px 0 6px 12px; }

  .main { padding: 16px 14px; }

  .hero h1 { font-size: 26px; }
  .lesson h2 { font-size: 24px; }

  .btn { min-height: 44px; font-size: 16px; padding: 10px 20px; }
  .exercise textarea { font-size: 16px; }

  .quiz .option { min-height: 44px; padding: 12px 14px; font-size: 16px; }
}