:root {
    --bg:#0d1117;--bg-card:#161b22;--bg-card-hover:#1c2230;--bg-inline:#1a2236;
    --border:#21293a;--border-accent:#2d3a2d;
    --blue-bright:#79b8ff;--blue-mid:#58a6ff;--blue-dim:#1f4e8c;
    --green:#56d364;--green-dim:#1a3d20;--green-bright:#a8f0c0;
    --red:#f85149;--red-dim:#3d1a1a;--red-bright:#ffa198;
    --yellow:#e3b341;--yellow-dim:#3d2e0a;
    --purple:#c084fc;--purple-dim:#2a1a4a;
    --amber:#f59e0b;--amber-dim:#2d1f00;--amber-bright:#fcd34d;
    --text-primary:#e6edf3;--text-secondary:#8b949e;--text-muted:#484f58;
    --sidebar-w:270px;
    --font-mono:'JetBrains Mono','Fira Code','Cascadia Code',monospace;
    --font-sans:-apple-system,BlinkMacSystemFont,'Segoe UI',sans-serif;
  }
  *{box-sizing:border-box;margin:0;padding:0}
  body{background:var(--bg);color:var(--text-primary);font-family:var(--font-sans);font-size:15px;line-height:1.7;display:flex;min-height:100vh}
  #sidebar{width:var(--sidebar-w);min-height:100vh;background:var(--bg-card);border-right:1px solid var(--border);position:fixed;top:0;left:0;overflow-y:auto;z-index:100;display:flex;flex-direction:column}
  .sidebar-header{padding:20px 18px 16px;border-bottom:1px solid var(--border)}
  .sidebar-course-tag{font-size:10px;font-weight:600;letter-spacing:.12em;text-transform:uppercase;color:var(--amber);margin-bottom:6px}
  .sidebar-title{font-size:13px;font-weight:600;color:var(--text-primary);line-height:1.4}
  .sidebar-progress-wrap{padding:12px 18px;border-bottom:1px solid var(--border)}
  .progress-label{font-size:11px;color:var(--text-secondary);margin-bottom:6px;display:flex;justify-content:space-between}
  .progress-bar{height:4px;background:var(--border);border-radius:2px;overflow:hidden}
  .progress-fill{height:100%;background:linear-gradient(90deg,var(--amber-dim),var(--amber));border-radius:2px;width:0%;transition:width .4s ease}
  .sidebar-nav{padding:10px 0;flex:1}
  .nav-section-label{font-size:10px;font-weight:600;letter-spacing:.1em;text-transform:uppercase;color:var(--text-muted);padding:10px 18px 4px}
  .nav-item{display:flex;align-items:center;gap:10px;padding:7px 18px;cursor:pointer;border-left:2px solid transparent;transition:all .15s;font-size:13px;color:var(--text-secondary)}
  .nav-item:hover{background:var(--bg-card-hover);color:var(--text-primary)}
  .nav-item.active{border-left-color:var(--amber);color:var(--amber);background:var(--amber-dim)}
  .nav-item.done{color:var(--green)}.nav-item.done .nav-dot{background:var(--green)}
  .nav-dot{width:7px;height:7px;border-radius:50%;background:var(--text-muted);flex-shrink:0}
  .nav-item.active .nav-dot{background:var(--amber);box-shadow:0 0 6px var(--amber)}
  #main{margin-left:var(--sidebar-w);flex:1;max-width:860px;padding:48px 52px 80px}
  .lesson-section{display:none;animation:fadeIn .25s ease}.lesson-section.active{display:block}
  @keyframes fadeIn{from{opacity:0;transform:translateY(8px)}to{opacity:1;transform:translateY(0)}}
  .lesson-eyebrow{font-size:11px;font-weight:600;letter-spacing:.12em;text-transform:uppercase;color:var(--amber);margin-bottom:10px}
  h1.lesson-title{font-size:34px;font-weight:700;line-height:1.2;letter-spacing:-.02em;color:var(--text-primary);margin-bottom:14px}
  .lesson-subtitle{font-size:16px;color:var(--text-secondary);line-height:1.6;margin-bottom:32px;max-width:620px}
  .lesson-meta{display:flex;gap:20px;margin-bottom:40px;padding-bottom:32px;border-bottom:1px solid var(--border)}
  .meta-pill{display:flex;align-items:center;gap:6px;font-size:12px;color:var(--text-secondary);background:var(--bg-card);border:1px solid var(--border);padding:4px 12px;border-radius:20px}
  .meta-pill .dot{width:6px;height:6px;border-radius:50%}
  h2{font-size:22px;font-weight:600;color:var(--text-primary);margin:36px 0 14px;letter-spacing:-.01em;display:flex;align-items:center;gap:10px}
  h2::before{content:'';display:block;width:3px;height:20px;background:var(--amber);border-radius:2px}
  h3{font-size:16px;font-weight:600;color:var(--amber);margin:24px 0 10px}
  p{margin-bottom:14px;color:var(--text-primary);line-height:1.75}
  .code-block{background:#090d13;border:1px solid var(--border);border-radius:8px;margin:16px 0 20px;overflow:hidden}
  .code-header{background:var(--bg-card);border-bottom:1px solid var(--border);padding:8px 16px;display:flex;align-items:center;justify-content:space-between}
  .code-filename{font-family:var(--font-mono);font-size:12px;color:var(--text-secondary)}
  .code-tag{font-size:10px;font-weight:600;letter-spacing:.08em;text-transform:uppercase;padding:2px 8px;border-radius:3px}
  .tag-bad{background:var(--red-dim);color:var(--red)}.tag-good{background:var(--green-dim);color:var(--green)}
  .tag-example{background:var(--blue-dim);color:var(--blue-bright)}.tag-pattern{background:var(--amber-dim);color:var(--amber)}
  pre{padding:18px 20px;overflow-x:auto;font-family:var(--font-mono);font-size:13.5px;line-height:1.65}
  .kw{color:#ff7b72}.type{color:#79b8ff}.method{color:#d2a8ff}
  .str{color:#a5d6ff}.num{color:#79c0ff}.comment{color:#484f58;font-style:italic}
  .anno{color:#f0883e}.iface{color:#f59e0b;font-weight:600}
  code{font-family:var(--font-mono);font-size:13px;background:var(--bg-inline);color:var(--amber);padding:2px 7px;border-radius:4px;border:1px solid var(--border)}
  .callout{border-radius:8px;padding:16px 18px;margin:18px 0;border-left:3px solid;font-size:14px;line-height:1.7}
  .callout-title{font-weight:600;font-size:12px;letter-spacing:.08em;text-transform:uppercase;margin-bottom:6px}
  .callout.info{background:#0c1929;border-color:var(--blue-mid)}.callout.info .callout-title{color:var(--blue-mid)}
  .callout.warning{background:var(--yellow-dim);border-color:var(--yellow)}.callout.warning .callout-title{color:var(--yellow)}
  .callout.tip{background:var(--green-dim);border-color:var(--green)}.callout.tip .callout-title{color:var(--green)}
  .callout.amber{background:var(--amber-dim);border-color:var(--amber)}.callout.amber .callout-title{color:var(--amber)}
  .callout.concept{background:var(--purple-dim);border-color:var(--purple)}.callout.concept .callout-title{color:var(--purple)}
  .analogy-card{background:linear-gradient(135deg,#1a1200 0%,#0d1117 100%);border:1px solid var(--border-accent);border-radius:10px;padding:22px 24px;margin:20px 0;position:relative;overflow:hidden}
  .analogy-card::before{content:'💡';position:absolute;top:18px;right:18px;font-size:22px;opacity:.5}
  .analogy-label{font-size:10px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;color:var(--purple);margin-bottom:8px}
  .analogy-card p{color:var(--text-secondary);margin-bottom:0}.analogy-card strong{color:var(--text-primary)}
  .concept-row{display:grid;grid-template-columns:repeat(3,1fr);gap:12px;margin:18px 0}
  .concept-tile{background:var(--bg-card);border:1px solid var(--border);border-radius:8px;padding:14px 16px;transition:border-color .2s}
  .concept-tile:hover{border-color:var(--amber)}
  .concept-tile-label{font-size:11px;font-weight:600;letter-spacing:.08em;text-transform:uppercase;color:var(--amber);margin-bottom:6px}
  .concept-tile-text{font-size:13px;color:var(--text-secondary);line-height:1.5}
  .compare-table{width:100%;border-collapse:collapse;margin:16px 0 20px;font-size:13.5px}
  .compare-table th{background:var(--bg-card);padding:10px 14px;text-align:left;font-size:11px;font-weight:600;letter-spacing:.08em;text-transform:uppercase;color:var(--text-secondary);border-bottom:1px solid var(--border)}
  .compare-table td{padding:10px 14px;border-bottom:1px solid var(--border);color:var(--text-primary);vertical-align:top;font-size:13.5px}
  .compare-table tr:last-child td{border-bottom:none}.compare-table tr:hover td{background:var(--bg-card-hover)}
  .td-bad{color:var(--red)}.td-good{color:var(--green)}
  .quiz-card{background:var(--bg-card);border:1px solid var(--border);border-radius:10px;padding:24px;margin:28px 0}
  .quiz-label{font-size:10px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;color:var(--yellow);margin-bottom:12px}
  .quiz-q{font-size:15px;font-weight:500;color:var(--text-primary);margin-bottom:16px;line-height:1.5}
  .quiz-options{display:flex;flex-direction:column;gap:8px}
  .quiz-option{background:var(--bg);border:1px solid var(--border);border-radius:7px;padding:10px 14px;cursor:pointer;font-size:13.5px;color:var(--text-secondary);transition:all .15s;display:flex;align-items:flex-start;gap:10px;text-align:left}
  .quiz-option:hover{border-color:var(--amber);color:var(--text-primary);background:var(--amber-dim)}
  .quiz-option.correct{border-color:var(--green);color:var(--green-bright);background:var(--green-dim)}
  .quiz-option.wrong{border-color:var(--red);color:var(--red-bright);background:var(--red-dim)}
  .option-letter{font-family:var(--font-mono);font-size:11px;font-weight:700;padding:1px 6px;border-radius:3px;background:var(--border);flex-shrink:0;margin-top:1px}
  .quiz-feedback{display:none;margin-top:14px;padding:12px 14px;border-radius:7px;font-size:13.5px;line-height:1.6}
  .quiz-feedback.show{display:block}
  .quiz-feedback.correct-fb{background:var(--green-dim);color:var(--green-bright);border:1px solid var(--green)}
  .quiz-feedback.wrong-fb{background:var(--red-dim);color:var(--red-bright);border:1px solid var(--red)}
  .challenge-card{background:linear-gradient(135deg,#0f1d12 0%,#0d1117 100%);border:1px solid #2a4a30;border-radius:10px;padding:24px;margin:28px 0}
  .challenge-label{font-size:10px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;color:var(--green);margin-bottom:10px}
  .challenge-title{font-size:16px;font-weight:600;color:var(--text-primary);margin-bottom:10px}
  .challenge-desc{color:var(--text-secondary);font-size:14px;margin-bottom:14px}
  .challenge-task{background:var(--bg);border:1px solid var(--border);border-radius:7px;padding:12px 14px;font-size:13.5px;color:var(--text-primary);margin-bottom:8px;display:flex;gap:10px;align-items:flex-start}
  .task-num{color:var(--green);font-family:var(--font-mono);font-size:12px;font-weight:600;flex-shrink:0;margin-top:2px}
  .reveal-btn{background:none;border:1px solid var(--border);color:var(--text-secondary);font-size:12px;padding:6px 14px;border-radius:5px;cursor:pointer;transition:all .15s;margin-top:8px;font-family:var(--font-sans)}
  .reveal-btn:hover{border-color:var(--green);color:var(--green)}
  .reveal-content{display:none;margin-top:12px}.reveal-content.show{display:block}
  .lesson-nav{display:flex;justify-content:space-between;align-items:center;margin-top:52px;padding-top:28px;border-top:1px solid var(--border)}
  .nav-btn{background:var(--bg-card);border:1px solid var(--border);color:var(--text-secondary);padding:10px 20px;border-radius:7px;cursor:pointer;font-size:13px;font-family:var(--font-sans);transition:all .15s}
  .nav-btn:hover{border-color:var(--amber);color:var(--amber)}
  .nav-btn:disabled{opacity:.3;cursor:not-allowed}
  .nav-btn.primary{background:var(--amber-dim);border-color:var(--amber);color:var(--amber);font-weight:500}
  .nav-btn.primary:hover{background:#3d2800}
  .section-indicator{font-size:12px;color:var(--text-muted)}
  .step-list{margin:14px 0}
  .step-item{display:flex;gap:14px;padding:12px 0;border-bottom:1px solid var(--border)}
  .step-item:last-child{border-bottom:none}
  .step-num{width:26px;height:26px;background:var(--amber-dim);color:var(--amber);border-radius:50%;display:flex;align-items:center;justify-content:center;font-size:12px;font-weight:700;flex-shrink:0;margin-top:2px}
  .step-content{flex:1}
  .step-title{font-weight:600;color:var(--text-primary);margin-bottom:3px;font-size:14px}
  .step-desc{color:var(--text-secondary);font-size:13.5px}

  /* PATTERN HEADER BADGE */
  .pattern-badge{display:inline-flex;align-items:center;gap:8px;background:var(--amber-dim);border:1px solid var(--amber);border-radius:6px;padding:6px 14px;margin-bottom:20px;font-size:12px;font-weight:600;color:var(--amber);letter-spacing:.06em;text-transform:uppercase}
  .pattern-badge .cat{color:var(--text-muted);font-weight:400;margin-left:4px}

  /* PATTERN INTENT BOX */
  .intent-box{background:linear-gradient(135deg,#1a1200 0%,#0d1117 100%);border:1px solid var(--amber-dim);border-radius:10px;padding:20px 22px;margin:16px 0 24px}
  .intent-label{font-size:10px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;color:var(--amber);margin-bottom:8px}
  .intent-text{font-size:15px;color:var(--text-primary);line-height:1.6;font-style:italic}

  /* PATTERN GRID */
  .pattern-overview-grid{display:grid;grid-template-columns:1fr 1fr 1fr;gap:14px;margin:20px 0}
  .pattern-card{background:var(--bg-card);border:1px solid var(--border);border-radius:10px;padding:18px}
  .pattern-card:hover{border-color:var(--amber)}
  .pc-cat{font-size:10px;font-weight:600;letter-spacing:.1em;text-transform:uppercase;margin-bottom:6px}
  .pc-cat.creational{color:var(--amber)}
  .pc-cat.structural{color:var(--blue-bright)}
  .pc-cat.behavioral{color:var(--purple)}
  .pc-name{font-size:15px;font-weight:600;color:var(--text-primary);margin-bottom:6px}
  .pc-desc{font-size:12px;color:var(--text-secondary);line-height:1.5}

  /* BEFORE/AFTER */
  .ba-grid{display:grid;grid-template-columns:1fr 1fr;gap:12px;margin:16px 0}

  /* SINGLETON VIZ */
  .singleton-viz{background:#090d13;border:1px solid var(--border);border-radius:8px;padding:20px;margin:16px 0;text-align:center}
  .sv-row{display:flex;justify-content:center;gap:16px;margin-bottom:12px}
  .sv-caller{background:var(--bg-card);border:1px solid var(--border);border-radius:6px;padding:8px 14px;font-family:var(--font-mono);font-size:12px;color:var(--text-secondary)}
  .sv-arrow{color:var(--text-muted);font-size:14px;align-self:center}
  .sv-instance{background:var(--amber-dim);border:2px solid var(--amber);border-radius:8px;padding:12px 24px;font-family:var(--font-mono);font-size:13px;color:var(--amber);font-weight:600}
  .sv-label{font-size:11px;color:var(--text-muted);margin-top:8px}

  /* BUILDER CHAIN */
  .builder-chain{background:#090d13;border:1px solid var(--border);border-radius:8px;padding:20px;margin:16px 0;overflow-x:auto}
  .bc-row{display:flex;align-items:center;gap:0;font-family:var(--font-mono);font-size:13px;flex-wrap:wrap;gap:4px}
  .bc-obj{color:var(--text-secondary)}
  .bc-method{color:var(--amber)}
  .bc-arg{color:var(--blue-bright)}
  .bc-dot{color:var(--text-muted)}

    .main { 
        width: 100% !important; 
        padding: 15px !important;
    }
    .lesson-nav { 
        flex-direction: column; 
        gap: 10px; 
    }
    button, .nav-btn, .btn { 
        min-height: 44px !important; 
        padding: 12px !important;
    }

/* Mobile Responsive Overrides */
@media (max-width: 768px) {
    .shell, .layout { flex-direction: column !important; }
    .sidebar { width: 100% !important; position: static !important; margin-bottom: 20px; }
    .main { width: 100% !important; padding: 15px !important; }
    .lesson-nav { flex-direction: column; gap: 10px; }
    button, .nav-btn, .btn { min-height: 44px !important; padding: 12px !important; }
    .concept-row, .pattern-overview-grid, .ba-grid { grid-template-columns: 1fr !important; }
}