:root {
    --bg: #0e1117;
    --surface: #161b26;
    --surface2: #1e2638;
    --border: #2a3347;
    --accent: #4f9cf9;
    --accent2: #f97316;
    --accent3: #22d3a5;
    --accent4: #c084fc;
    --accent5: #fb7185;
    --text: #e2e8f0;
    --text2: #94a3b8;
    --text3: #4a5568;
    --kw: #c084fc;
    --fn: #22d3a5;
    --num: #fb923c;
    --str: #f472b6;
    --cm: #4a6080;
    --op: #fbbf24;
    --var: #93c5fd;
  }

  * { box-sizing: border-box; margin: 0; padding: 0; }

  body {
    background: var(--bg);
    color: var(--text);
    font-family: 'DM Sans', sans-serif;
    min-height: 100vh;
    padding: 0;
  }

  /* Header */
  header {
    background: var(--surface);
    border-bottom: 1px solid var(--border);
    padding: 1.5rem 2rem;
    display: flex;
    align-items: center;
    gap: 1rem;
    position: sticky;
    top: 0;
    z-index: 100;
  }
  .header-badge {
    background: linear-gradient(135deg, #4f9cf9, #c084fc);
    color: #fff;
    font-size: 11px;
    font-weight: 600;
    letter-spacing: 0.08em;
    padding: 3px 10px;
    border-radius: 99px;
    text-transform: uppercase;
  }
  header h1 {
    font-size: 18px;
    font-weight: 600;
    color: var(--text);
  }
  header p {
    font-size: 13px;
    color: var(--text2);
    margin-top: 1px;
  }
  .header-text { flex: 1; }
  .day-nav { display: flex; gap: 6px; flex-wrap: wrap; }
  .day-link {
    font-family: 'DM Sans', sans-serif;
    font-size: 12px;
    font-weight: 500;
    padding: 5px 12px;
    border-radius: 6px;
    border: 1px solid var(--border);
    background: var(--surface2);
    color: var(--text2);
    text-decoration: none;
    transition: all 0.15s;
  }
  .day-link:hover { color: var(--text); border-color: var(--accent); }
  .day-link.current { color: var(--accent); border-color: var(--accent); background: rgba(79,156,249,0.12); }
  .day-link.home-link { color: var(--accent3); border-color: var(--accent3); background: rgba(34,211,165,0.08); }
  .day-link.home-link:hover { border-color: var(--accent3); color: var(--text); }

  /* Layout */
  .container { max-width: 980px; margin: 0 auto; padding: 2rem; }

  /* Tab nav */
  .tab-nav {
    display: flex;
    gap: 6px;
    flex-wrap: wrap;
    margin-bottom: 2rem;
    background: var(--surface);
    padding: 6px;
    border-radius: 12px;
    border: 1px solid var(--border);
  }
  .tab-btn {
    font-family: 'DM Sans', sans-serif;
    font-size: 13px;
    font-weight: 500;
    padding: 7px 16px;
    border-radius: 8px;
    border: none;
    background: transparent;
    color: var(--text2);
    cursor: pointer;
    transition: all 0.15s;
    white-space: nowrap;
  }
  .tab-btn:hover { background: var(--surface2); color: var(--text); }
  .tab-btn.active { background: var(--accent); color: #fff; }

  /* Lesson */
  .lesson { display: none; }
  .lesson.active { display: block; animation: fadeIn 0.2s ease; }
  @keyframes fadeIn { from { opacity: 0; transform: translateY(6px); } to { opacity: 1; transform: translateY(0); } }

  /* Cards */
  .card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 1.25rem 1.5rem;
    margin-bottom: 1rem;
  }
  .card-label {
    font-size: 10px;
    font-weight: 600;
    letter-spacing: 0.1em;
    text-transform: uppercase;
    color: var(--text3);
    margin-bottom: 10px;
    display: flex;
    align-items: center;
    gap: 6px;
  }
  .card-label span { color: var(--accent); }

  /* Scenario box */
  .scenario {
    background: var(--surface2);
    border-left: 3px solid var(--accent);
    border-radius: 0 8px 8px 0;
    padding: 10px 14px;
    font-size: 14px;
    line-height: 1.7;
    color: var(--text2);
    margin-bottom: 1rem;
  }

  /* Sliders */
  .slider-row {
    display: flex;
    align-items: center;
    gap: 12px;
    margin: 8px 0;
  }
  .slider-row label {
    font-size: 13px;
    color: var(--text2);
    min-width: 160px;
  }
  .slider-row input[type=range] {
    flex: 1;
    height: 4px;
    -webkit-appearance: none;
    appearance: none;
    background: var(--border);
    border-radius: 99px;
    outline: none;
    cursor: pointer;
  }
  .slider-row input[type=range]::-webkit-slider-thumb {
    -webkit-appearance: none;
    width: 16px;
    height: 16px;
    border-radius: 50%;
    background: var(--accent);
    cursor: pointer;
  }
  .slider-val {
    font-family: 'JetBrains Mono', monospace;
    font-size: 13px;
    font-weight: 500;
    min-width: 80px;
    text-align: right;
    color: var(--accent);
  }

  /* Code block */
  .code-wrap {
    background: #0a0e17;
    border: 1px solid var(--border);
    border-radius: 10px;
    overflow: hidden;
  }
  .code-topbar {
    background: var(--surface2);
    border-bottom: 1px solid var(--border);
    padding: 7px 14px;
    display: flex;
    align-items: center;
    gap: 6px;
  }
  .dot { width: 10px; height: 10px; border-radius: 50%; }
  .dot-r { background: #ff5f56; }
  .dot-y { background: #ffbd2e; }
  .dot-g { background: #27c93f; }
  .code-filename {
    font-family: 'JetBrains Mono', monospace;
    font-size: 11px;
    color: var(--text3);
    margin-left: 6px;
  }
  .copy-btn {
    margin-left: auto;
    border: 1px solid var(--border);
    background: rgba(79, 156, 249, 0.12);
    color: #b8d5ff;
    font-family: 'DM Sans', sans-serif;
    font-size: 11px;
    font-weight: 600;
    border-radius: 6px;
    padding: 0.18rem 0.5rem;
    cursor: pointer;
  }
  .copy-btn:hover {
    border-color: var(--accent);
    color: var(--text);
  }
  .copy-btn.copied {
    border-color: var(--accent3);
    color: var(--accent3);
    background: rgba(34, 211, 165, 0.12);
  }
  pre {
    font-family: 'JetBrains Mono', monospace;
    font-size: 13px;
    line-height: 1.9;
    padding: 1.2rem 1.5rem;
    overflow-x: auto;
    color: var(--text);
    white-space: pre;
  }

  /* Output */
  .output-wrap {
    background: #0a0e17;
    border: 1px solid var(--border);
    border-radius: 10px;
    overflow: hidden;
  }
  .output-topbar {
    background: var(--surface2);
    border-bottom: 1px solid var(--border);
    padding: 7px 14px;
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 11px;
    color: var(--text3);
  }
  .live-dot {
    width: 7px; height: 7px; border-radius: 50%;
    background: var(--accent3);
    animation: pulse 1.5s infinite;
  }
  @keyframes pulse { 0%,100%{opacity:1} 50%{opacity:0.4} }
  .output-lines {
    padding: 1rem 1.5rem;
    font-family: 'JetBrains Mono', monospace;
    font-size: 13px;
    line-height: 2;
  }
  .out-line {
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-bottom: 1px solid #111827;
    padding: 2px 0;
  }
  .out-line:last-child { border-bottom: none; }
  .out-val { color: var(--accent3); }
  .out-explain { color: var(--text3); font-size: 11px; }

  /* Operator pills */
  .ops {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
    margin-top: 10px;
  }
  .op-pill {
    font-family: 'JetBrains Mono', monospace;
    font-size: 12px;
    padding: 4px 12px;
    border-radius: 99px;
    border: 1px solid;
  }
  .op-mod   { color: var(--op);     border-color: var(--op);     background: rgba(251,191,36,0.08); }
  .op-div   { color: var(--accent2);border-color: var(--accent2);background: rgba(249,115,22,0.08); }
  .op-fstr  { color: var(--str);    border-color: var(--str);    background: rgba(244,114,182,0.08); }
  .op-mul   { color: var(--accent); border-color: var(--accent); background: rgba(79,156,249,0.08); }
  .op-fmt   { color: var(--accent4);border-color: var(--accent4);background: rgba(192,132,252,0.08); }
  .op-var   { color: var(--var);    border-color: var(--var);    background: rgba(147,197,253,0.08); }

  /* Syntax highlight classes */
  .kw  { color: var(--kw); }
  .fn  { color: var(--fn); }
  .nm  { color: var(--num); }
  .st  { color: var(--str); }
  .cm  { color: var(--cm); font-style: italic; }
  .op  { color: var(--op); font-weight: 700; }
  .vr  { color: var(--var); }

  /* Two-col grid */
  .grid2 {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1rem;
  }
  @media (max-width: 640px) { .grid2 { grid-template-columns: 1fr; } }
  @media (max-width: 880px) {
    header { flex-wrap: wrap; align-items: flex-start; }
    .day-nav { width: 100%; }
  }

  /* Section title */
  .section-title {
    font-size: 20px;
    font-weight: 600;
    margin-bottom: 4px;
    color: var(--text);
  }
  .section-desc {
    font-size: 14px;
    color: var(--text2);
    margin-bottom: 1.5rem;
    line-height: 1.6;
  }

  /* Concept callout */
  .concept {
    display: flex;
    gap: 10px;
    background: var(--surface2);
    border: 1px solid var(--border);
    border-radius: 10px;
    padding: 10px 14px;
    margin-bottom: 1rem;
    align-items: flex-start;
  }
  .concept-icon {
    font-size: 18px;
    margin-top: 1px;
    flex-shrink: 0;
  }
  .concept-text { font-size: 13px; color: var(--text2); line-height: 1.6; }
  .concept-text strong { color: var(--text); }

  footer {
    text-align: center;
    padding: 2rem;
    font-size: 12px;
    color: var(--text3);
    border-top: 1px solid var(--border);
    margin-top: 2rem;
  }