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

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

  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, #22d3a5, #4f9cf9);
    color: #0e1117; font-size: 11px; font-weight: 700;
    letter-spacing: 0.08em; padding: 3px 10px;
    border-radius: 99px; text-transform: uppercase;
  }
  header h1 { font-size: 18px; font-weight: 600; }
  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(34,211,165,0.12); }
  .day-link.home-link { color: var(--accent3); border-color: var(--accent3); background: rgba(79,156,249,0.08); }
  .day-link.home-link:hover { border-color: var(--accent3); color: var(--text); }

  .container { max-width: 1020px; margin: 0 auto; padding: 2rem; }

  .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: #0e1117; }

  .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); } }

  .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;
  }
  .section-title { font-size: 20px; font-weight: 600; margin-bottom: 4px; }
  .section-desc  { font-size: 14px; color: var(--text2); margin-bottom: 1.25rem; line-height: 1.6; }

  .concept {
    display: flex; gap: 10px;
    background: var(--surface2); border: 1px solid var(--border);
    border-radius: 10px; padding: 10px 14px; margin-bottom: 1rem;
  }
  .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); }

  .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;
  }

  .slider-row { display: flex; align-items: center; gap: 12px; margin: 8px 0; }
  .slider-row label { font-size: 13px; color: var(--text2); min-width: 180px; }
  .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-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: 12.5px; line-height: 1.9;
    padding: 1.2rem 1.5rem; overflow-x: auto; color: var(--text); white-space: pre;
  }

  .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(--accent); 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: 12.5px; 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(--accent); }
  .out-explain { color: var(--text3); font-size: 11px; }

  .grid2 { display: grid; grid-template-columns: 1fr 1fr; gap: 1rem; }
  @media(max-width:680px) { .grid2 { grid-template-columns: 1fr; } }
  @media(max-width:880px) {
    header { flex-wrap: wrap; align-items: flex-start; }
    .day-nav { width: 100%; }
  }

  .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-def   { color: var(--dec);    border-color: var(--dec);    background: rgba(251,113,133,0.08); }
  .op-list  { color: var(--accent3);border-color: var(--accent3);background: rgba(79,156,249,0.08); }
  .op-dict  { color: var(--op);     border-color: var(--op);     background: rgba(251,191,36,0.08); }
  .op-str   { color: var(--str);    border-color: var(--str);    background: rgba(244,114,182,0.08); }
  .op-try   { color: var(--accent5);border-color: var(--accent5);background: rgba(251,113,133,0.08); }
  .op-file  { color: var(--accent4);border-color: var(--accent4);background: rgba(192,132,252,0.08); }
  .op-ret   { color: var(--fn);     border-color: var(--fn);     background: rgba(34,211,165,0.08); }

  /* Syntax colors */
  .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); }
  .dc { color: var(--dec); }

  /* Interactive text input */
  .txt-input {
    font-family: 'JetBrains Mono', monospace; font-size: 13px;
    background: #0a0e17; border: 1px solid var(--border); border-radius: 8px;
    color: var(--text); padding: 8px 12px; width: 100%; outline: none;
    transition: border-color 0.15s;
  }
  .txt-input:focus { border-color: var(--accent); }

  /* Tag chips */
  .chip {
    display: inline-block; font-family: 'JetBrains Mono', monospace;
    font-size: 12px; padding: 3px 10px; border-radius: 6px;
    background: var(--surface2); border: 1px solid var(--border);
    color: var(--text2); margin: 2px;
  }

  /* Data table inside output */
  .data-table { width: 100%; border-collapse: collapse; font-family: 'JetBrains Mono', monospace; font-size: 12px; }
  .data-table th { color: var(--text3); font-weight: 500; padding: 6px 12px; border-bottom: 1px solid var(--border); text-align: left; }
  .data-table td { padding: 6px 12px; border-bottom: 1px solid #111827; color: var(--text2); }
  .data-table td.hi { color: var(--accent); }
  .data-table tr:last-child td { border-bottom: none; }

  /* Error styling */
  .error-line { color: var(--accent5); }
  .success-line { color: var(--accent); }

  /* File preview */
  .file-preview {
    background: #060a10; border: 1px solid var(--border); border-radius: 8px;
    padding: 10px 14px; font-family: 'JetBrains Mono', monospace; font-size: 12px;
    color: var(--text2); line-height: 1.8; white-space: pre-wrap;
  }

  /* Tag labels */
  .tag { font-size: 11px; padding: 2px 8px; border-radius: 4px; font-family: 'JetBrains Mono', monospace; }
  .tag-def  { background: rgba(251,113,133,0.15); color: var(--dec); }
  .tag-new  { background: rgba(34,211,165,0.15);  color: var(--accent); }

  /* btn */
  .btn {
    font-family: 'DM Sans', sans-serif; font-size: 12px; padding: 5px 14px;
    border-radius: 6px; border: 1px solid var(--border);
    background: var(--surface2); color: var(--text2); cursor: pointer;
    transition: all 0.15s;
  }
  .btn:hover { background: var(--surface); color: var(--text); border-color: var(--accent); }

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