:root {
      --green: #6aaa4b;
      --green-dark: #4a8a2e;
      --green-light: #e8f5e1;
      --sidebar-bg: #1a2332;
      --sidebar-text: #c9d6e3;
      --sidebar-hover: #2d3f55;
      --sidebar-active: #6aaa4b;
      --body-bg: #f5f7fa;
      --card-bg: #ffffff;
      --text: #2c3e50;
      --text-light: #5a6a7a;
      --code-bg: #282c34;
      --border: #e0e7ef;
      --warning-bg: #fff8e1;
      --warning-border: #f59e0b;
      --tip-bg: #e8f5e1;
      --tip-border: #6aaa4b;
      --danger-bg: #fef2f2;
      --danger-border: #ef4444;
      --info-bg: #eff6ff;
      --info-border: #3b82f6;
      --exercise-bg: #f0fdf4;
      --exercise-border: #22c55e;
    }
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body { font-family: 'Segoe UI', system-ui, sans-serif; background: var(--body-bg); color: var(--text); display: flex; flex-direction: column; min-height: 100vh; }
    #layout-row { display: flex; flex: 1; min-height: 0; align-items: stretch; width: 100%; }
    #layout-row #main { min-width: 0; }

    /* SIDEBAR */
    #sidebar {
      width: 300px; min-width: 300px; background: var(--sidebar-bg);
      color: var(--sidebar-text); height: 100vh; position: sticky; top: 0;
      overflow-y: auto; display: flex; flex-direction: column;
    }
    #sidebar::-webkit-scrollbar { width: 6px; }
    #sidebar::-webkit-scrollbar-track { background: #111; }
    #sidebar::-webkit-scrollbar-thumb { background: #444; border-radius: 3px; }
    .sidebar-header { padding: 24px 20px 16px; border-bottom: 1px solid #2d3f55; }
    .sidebar-title { font-size: 14px; font-weight: 700; color: var(--green); text-transform: uppercase; letter-spacing: 1px; }
    .sidebar-subtitle { font-size: 11px; color: #7a8fa6; margin-top: 4px; }
    .sidebar-nav { padding: 12px 0; flex: 1; }
    .nav-part { padding: 8px 20px 4px; font-size: 10px; font-weight: 700; color: #4a6480; text-transform: uppercase; letter-spacing: 1.5px; }
    .nav-item { display: block; padding: 7px 20px 7px 28px; font-size: 13px; color: var(--sidebar-text); text-decoration: none; border-left: 3px solid transparent; transition: all .15s; }
    .nav-item:hover { background: var(--sidebar-hover); color: #fff; border-left-color: var(--green); }
    .nav-item.active { background: #1f3048; color: #fff; border-left-color: var(--green); font-weight: 600; }
    .nav-item .cap-num { color: var(--green); font-weight: 700; margin-right: 6px; font-size: 11px; }

    /* MAIN */
    #main { flex: 1; max-width: 900px; margin: 0 auto; padding: 40px 48px; }
    @media (max-width: 1100px) { #main { padding: 24px 24px; } #sidebar { width: 260px; min-width: 260px; } }

    /* COVER */
    .cover { text-align: center; padding: 80px 40px; background: linear-gradient(135deg, #1a2332 0%, #2d3f55 100%); color: white; border-radius: 16px; margin-bottom: 48px; }
    .cover-badge { display: inline-block; background: var(--green); color: white; font-size: 11px; font-weight: 700; padding: 4px 12px; border-radius: 20px; text-transform: uppercase; letter-spacing: 1px; margin-bottom: 20px; }
    .cover h1 { font-size: 38px; font-weight: 800; line-height: 1.2; margin-bottom: 12px; }
    .cover h1 span { color: var(--green); }
    .cover-sub { font-size: 18px; color: #a0b4c8; margin-bottom: 32px; }
    .cover-meta { display: flex; justify-content: center; gap: 32px; flex-wrap: wrap; }
    .cover-meta-item { font-size: 13px; color: #7a8fa6; }
    .cover-meta-item strong { color: #c9d6e3; display: block; font-size: 15px; margin-bottom: 2px; }

    /* TOC */
    .toc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
    @media (max-width: 700px) { .toc-grid { grid-template-columns: 1fr; } }
    .toc-part { grid-column: 1 / -1; background: var(--green-light); border-left: 4px solid var(--green); padding: 10px 16px; border-radius: 0 6px 6px 0; margin-top: 8px; }
    .toc-part h3 { font-size: 13px; font-weight: 700; color: var(--green-dark); text-transform: uppercase; letter-spacing: .5px; }
    .toc-item { display: flex; align-items: baseline; gap: 8px; padding: 8px 14px; background: white; border: 1px solid var(--border); border-radius: 8px; text-decoration: none; color: var(--text); transition: all .15s; }
    .toc-item:hover { border-color: var(--green); box-shadow: 0 2px 8px rgba(106,170,75,.15); transform: translateY(-1px); }
    .toc-item .toc-num { font-size: 11px; font-weight: 800; color: var(--green); min-width: 28px; }
    .toc-item .toc-title { font-size: 13px; }

    /* CHAPTERS */
    .chapter { display: none; }
    .chapter.active { display: block; }
    .chapter-header { margin-bottom: 32px; padding-bottom: 24px; border-bottom: 2px solid var(--border); }
    .chapter-part { font-size: 11px; font-weight: 700; color: var(--green); text-transform: uppercase; letter-spacing: 1px; margin-bottom: 8px; }
    .chapter-num { font-size: 13px; color: var(--text-light); margin-bottom: 6px; }
    .chapter-title { font-size: 30px; font-weight: 800; color: var(--text); line-height: 1.2; }
    .chapter-intro { font-size: 16px; color: var(--text-light); margin-top: 12px; line-height: 1.7; }

    h2.section { font-size: 20px; font-weight: 700; color: var(--text); margin: 36px 0 14px; padding-bottom: 8px; border-bottom: 1px solid var(--border); }
    h3.subsection { font-size: 16px; font-weight: 700; color: var(--text); margin: 24px 0 10px; }
    p { font-size: 15px; line-height: 1.8; color: var(--text); margin-bottom: 14px; }
    ul, ol { margin: 10px 0 16px 24px; }
    li { font-size: 15px; line-height: 1.8; margin-bottom: 4px; }
    strong { color: var(--text); }
    code { font-family: 'Fira Code', 'Cascadia Code', monospace; font-size: 13px; background: #eef1f6; padding: 2px 6px; border-radius: 4px; color: #c0392b; }

    /* CODE BLOCKS */
    .code-block { margin: 20px 0; border-radius: 10px; overflow: hidden; box-shadow: 0 2px 12px rgba(0,0,0,.12); }
    .code-header { background: #21252b; padding: 8px 16px; display: flex; align-items: center; justify-content: space-between; }
    .code-filename { font-size: 12px; color: #abb2bf; font-family: monospace; }
    .code-lang { font-size: 10px; color: #5a6a7a; background: #2d3f55; padding: 2px 8px; border-radius: 10px; text-transform: uppercase; letter-spacing: .5px; }
    .code-block pre { margin: 0 !important; border-radius: 0 !important; }
    .code-block pre code { font-size: 13px !important; line-height: 1.6 !important; }

    /* MERMAID */
    .diagram-wrap { background: white; border: 1px solid var(--border); border-radius: 10px; padding: 24px; margin: 24px 0; text-align: center; }
    .diagram-title { font-size: 12px; color: var(--text-light); margin-bottom: 12px; font-weight: 600; text-transform: uppercase; letter-spacing: .5px; }

    /* CALLOUT BOXES */
    .box { padding: 16px 20px; border-radius: 8px; margin: 20px 0; border-left: 4px solid; }
    .box-title { font-size: 12px; font-weight: 800; text-transform: uppercase; letter-spacing: .5px; margin-bottom: 6px; }
    .box p, .box li { font-size: 14px; margin-bottom: 6px; }
    .box ul { margin: 6px 0 0 18px; }
    .box-tip { background: var(--tip-bg); border-color: var(--tip-border); }
    .box-tip .box-title { color: var(--green-dark); }
    .box-warning { background: var(--warning-bg); border-color: var(--warning-border); }
    .box-warning .box-title { color: #92400e; }
    .box-danger { background: var(--danger-bg); border-color: var(--danger-border); }
    .box-danger .box-title { color: #991b1b; }
    .box-info { background: var(--info-bg); border-color: var(--info-border); }
    .box-info .box-title { color: #1e40af; }

    /* EXERCISE */
    .exercise { background: var(--exercise-bg); border: 2px solid var(--exercise-border); border-radius: 12px; padding: 20px 24px; margin: 32px 0; }
    .exercise-header { display: flex; align-items: center; gap: 10px; margin-bottom: 14px; }
    .exercise-icon { background: var(--exercise-border); color: white; width: 28px; height: 28px; border-radius: 50%; display: flex; align-items: center; justify-content: center; font-size: 14px; font-weight: 800; flex-shrink: 0; }
    .exercise-title { font-size: 15px; font-weight: 700; color: #15803d; }
    .exercise h4 { font-size: 13px; font-weight: 700; color: #15803d; margin: 12px 0 6px; text-transform: uppercase; letter-spacing: .4px; }
    .exercise p, .exercise li { font-size: 14px; }
    .exercise ul { margin: 4px 0 8px 20px; }

    /* WHAT WE LEARNED */
    .learned { background: var(--sidebar-bg); color: var(--sidebar-text); border-radius: 10px; padding: 20px 24px; margin: 32px 0; }
    .learned-title { font-size: 13px; font-weight: 700; color: var(--green); text-transform: uppercase; letter-spacing: 1px; margin-bottom: 12px; }
    .learned ul { margin-left: 18px; }
    .learned li { font-size: 14px; line-height: 1.7; color: #c9d6e3; margin-bottom: 4px; }

    /* SOURCE */
    .source { font-size: 12px; color: var(--text-light); border-top: 1px solid var(--border); padding-top: 12px; margin-top: 8px; }
    .source a { color: var(--green); text-decoration: none; }
    .source a:hover { text-decoration: underline; }

    /* NAVIGATION BUTTONS */
    .chapter-nav { display: flex; justify-content: space-between; margin-top: 48px; padding-top: 24px; border-top: 1px solid var(--border); gap: 12px; }
    .nav-btn { display: flex; align-items: center; gap: 8px; padding: 12px 20px; border: 2px solid var(--border); border-radius: 10px; text-decoration: none; color: var(--text); font-size: 14px; font-weight: 600; transition: all .15s; max-width: 48%; }
    .nav-btn:hover { border-color: var(--green); color: var(--green-dark); background: var(--green-light); }
    .nav-btn.next { margin-left: auto; flex-direction: row-reverse; text-align: right; }
    .nav-btn .nav-label { font-size: 11px; color: var(--text-light); display: block; font-weight: 400; }
    .nav-arrow { font-size: 18px; color: var(--green); flex-shrink: 0; }

    /* FOOTER */
    footer { background: var(--sidebar-bg); color: var(--sidebar-text); padding: 20px 40px; text-align: center; font-size: 13px; border-top: 3px solid var(--green); margin-top: 0; width: 100%; flex-shrink: 0; box-sizing: border-box; }
    footer .footer-brand { font-weight: 700; color: var(--green); font-size: 15px; justify-content: center}
    footer .footer-meta { color: #5a6a7a; margin-top: 4px; font-size: 12px; }

    /* TABLE */
    table { width: 100%; border-collapse: collapse; margin: 16px 0; font-size: 14px; }
    th { background: var(--sidebar-bg); color: var(--green); padding: 10px 14px; text-align: left; font-size: 12px; text-transform: uppercase; letter-spacing: .5px; }
    td { padding: 10px 14px; border-bottom: 1px solid var(--border); }
    tr:nth-child(even) { background: #f8fafc; }
    tr:hover { background: var(--green-light); }

    /* PAGE: COVER/TOC */
    #page-cover { display: block; }
    #page-toc { display: none; }