@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;500;600&family=Fraunces:ital,wght@0,300;0,600;1,300&display=swap');

  :root {
    --spring-green: #6DB33F;
    --spring-dark: #3D6B22;
    --spring-light: #8FC45A;
    --bg-primary: #0F1117;
    --bg-secondary: #161B22;
    --bg-card: #1C2128;
    --bg-elevated: #21262D;
    --border: #30363D;
    --border-subtle: #21262D;
    --text-primary: #E6EDF3;
    --text-secondary: #8B949E;
    --text-muted: #484F58;
    --accent-blue: #388BFD;
    --accent-orange: #F0883E;
    --accent-purple: #BC8CFF;
    --accent-red: #FA7970;
    --sidebar-w: 280px;
  }

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

  body {
    font-family: 'Inter', sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    display: flex;
    min-height: 100vh;
    font-size: 15px;
    line-height: 1.6;
  }

  /* === SIDEBAR === */
  #sidebar {
    width: var(--sidebar-w);
    background: var(--bg-secondary);
    border-right: 1px solid var(--border);
    display: flex;
    flex-direction: column;
    position: fixed;
    top: 0; left: 0; bottom: 0;
    overflow-y: auto;
    z-index: 100;
  }

  .sidebar-brand {
    padding: 20px 20px 16px;
    border-bottom: 1px solid var(--border);
  }

  .brand-logo {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 8px;
  }

  .spring-leaf {
    width: 32px;
    height: 32px;
  }

  .brand-title {
    font-family: 'Fraunces', serif;
    font-size: 17px;
    font-weight: 600;
    color: var(--spring-green);
    letter-spacing: -0.3px;
  }

  .brand-sub {
    font-size: 11px;
    color: var(--text-muted);
    font-family: 'JetBrains Mono', monospace;
    letter-spacing: 0.5px;
    text-transform: uppercase;
  }

  .progress-bar-wrap {
    padding: 12px 20px;
    border-bottom: 1px solid var(--border);
  }

  .progress-label {
    display: flex;
    justify-content: space-between;
    font-size: 11px;
    color: var(--text-muted);
    margin-bottom: 6px;
    font-family: 'JetBrains Mono', monospace;
  }

  .progress-track {
    height: 3px;
    background: var(--bg-elevated);
    border-radius: 2px;
    overflow: hidden;
  }

  .progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--spring-dark), var(--spring-green));
    border-radius: 2px;
    transition: width 0.4s ease;
    width: 0%;
  }

  .sidebar-section {
    padding: 12px 0;
  }

  .section-label {
    padding: 4px 20px 8px;
    font-size: 10px;
    font-family: 'JetBrains Mono', monospace;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 1px;
  }

  .nav-item {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 8px 20px;
    cursor: pointer;
    transition: background 0.15s;
    border-left: 3px solid transparent;
    font-size: 13px;
    color: var(--text-secondary);
  }

  .nav-item:hover { background: var(--bg-elevated); color: var(--text-primary); }

  .nav-item.active {
    background: rgba(109, 179, 63, 0.08);
    border-left-color: var(--spring-green);
    color: var(--spring-light);
  }

  .nav-item.completed { color: var(--text-secondary); }
  .nav-item.completed .nav-check { color: var(--spring-green); }

  .nav-num {
    font-family: 'JetBrains Mono', monospace;
    font-size: 10px;
    color: var(--text-muted);
    min-width: 20px;
  }

  .nav-check { font-size: 12px; min-width: 20px; text-align: center; }

  /* === MAIN === */
  #main {
    margin-left: var(--sidebar-w);
    flex: 1;
    min-height: 100vh;
  }

  .topbar {
    background: var(--bg-secondary);
    border-bottom: 1px solid var(--border);
    padding: 12px 40px;
    display: flex;
    align-items: center;
    justify-content: space-between;
    position: sticky;
    top: 0;
    z-index: 50;
  }

  .topbar-breadcrumb {
    font-size: 12px;
    color: var(--text-muted);
    font-family: 'JetBrains Mono', monospace;
  }

  .topbar-breadcrumb span { color: var(--spring-green); }

  .topbar-actions { display: flex; gap: 8px; }

  .btn {
    padding: 6px 14px;
    border-radius: 6px;
    font-size: 12px;
    font-weight: 500;
    cursor: pointer;
    border: 1px solid var(--border);
    background: var(--bg-elevated);
    color: var(--text-secondary);
    transition: all 0.15s;
    font-family: 'Inter', sans-serif;
  }

  .btn:hover { color: var(--text-primary); border-color: var(--text-muted); }

  .btn-primary {
    background: var(--spring-green);
    border-color: var(--spring-green);
    color: #fff;
  }

  .btn-primary:hover { background: var(--spring-light); border-color: var(--spring-light); color: #fff; }

  /* === LESSON CONTENT === */
  .lesson-wrap {
    max-width: 860px;
    margin: 0 auto;
    padding: 48px 40px 80px;
  }

  .lesson-eyebrow {
    font-family: 'JetBrains Mono', monospace;
    font-size: 11px;
    color: var(--spring-green);
    text-transform: uppercase;
    letter-spacing: 1.5px;
    margin-bottom: 12px;
  }

  .lesson-title {
    font-family: 'Fraunces', serif;
    font-size: 36px;
    font-weight: 600;
    line-height: 1.2;
    letter-spacing: -0.5px;
    margin-bottom: 16px;
    color: var(--text-primary);
  }

  .lesson-lead {
    font-size: 17px;
    color: var(--text-secondary);
    line-height: 1.7;
    margin-bottom: 40px;
    padding-bottom: 32px;
    border-bottom: 1px solid var(--border-subtle);
  }

  h2 {
    font-family: 'Fraunces', serif;
    font-size: 22px;
    font-weight: 600;
    color: var(--text-primary);
    margin: 36px 0 14px;
    letter-spacing: -0.3px;
  }

  h3 {
    font-size: 15px;
    font-weight: 600;
    color: var(--text-primary);
    margin: 24px 0 10px;
  }

  p { margin-bottom: 14px; color: var(--text-secondary); }

  /* === CODE BLOCKS === */
  .code-block {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: 8px;
    margin: 20px 0;
    overflow: hidden;
  }

  .code-header {
    background: var(--bg-elevated);
    padding: 8px 16px;
    display: flex;
    align-items: center;
    justify-content: space-between;
    border-bottom: 1px solid var(--border);
  }

  .code-filename {
    font-family: 'JetBrains Mono', monospace;
    font-size: 11px;
    color: var(--text-secondary);
  }

  .code-lang {
    font-family: 'JetBrains Mono', monospace;
    font-size: 10px;
    color: var(--text-muted);
    background: var(--bg-primary);
    padding: 2px 8px;
    border-radius: 4px;
    border: 1px solid var(--border);
  }

  pre {
    padding: 20px;
    overflow-x: auto;
    font-family: 'JetBrains Mono', monospace;
    font-size: 13px;
    line-height: 1.7;
    color: var(--text-primary);
  }

  .kw { color: var(--accent-purple); }
  .ann { color: var(--accent-orange); }
  .str { color: var(--spring-light); }
  .num { color: var(--accent-blue); }
  .cm { color: var(--text-muted); font-style: italic; }
  .cls { color: #79c0ff; }
  .fn { color: #d2a8ff; }
  .ret { color: var(--accent-orange); }

  /* === CALLOUTS === */
  .callout {
    border-radius: 8px;
    padding: 16px 20px;
    margin: 20px 0;
    border-left: 4px solid;
    display: flex;
    gap: 12px;
  }

  .callout-info { background: rgba(56, 139, 253, 0.08); border-color: var(--accent-blue); }
  .callout-warn { background: rgba(240, 136, 62, 0.08); border-color: var(--accent-orange); }
  .callout-tip  { background: rgba(109, 179, 63, 0.08); border-color: var(--spring-green); }
  .callout-key  { background: rgba(188, 140, 255, 0.08); border-color: var(--accent-purple); }

  .callout-icon { font-size: 16px; line-height: 1.6; }
  .callout-body { flex: 1; }
  .callout-title { font-weight: 600; font-size: 13px; margin-bottom: 4px; color: var(--text-primary); }
  .callout p { margin-bottom: 0; font-size: 13.5px; }

  /* === ARCHITECTURE DIAGRAM === */
  .arch-diagram {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 28px;
    margin: 24px 0;
    font-family: 'JetBrains Mono', monospace;
    font-size: 12px;
    overflow-x: auto;
  }

  /* === CONCEPT GRID === */
  .concept-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 12px;
    margin: 20px 0;
  }

  .concept-card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 16px;
    transition: border-color 0.15s;
  }

  .concept-card:hover { border-color: var(--spring-green); }

  .concept-card-icon { font-size: 20px; margin-bottom: 8px; }
  .concept-card-title { font-weight: 600; font-size: 13px; margin-bottom: 6px; color: var(--text-primary); }
  .concept-card p { font-size: 12.5px; margin: 0; }

  /* === QUIZ === */
  .quiz-section {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: 10px;
    padding: 28px;
    margin: 36px 0;
  }

  .quiz-header {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 20px;
  }

  .quiz-badge {
    background: var(--spring-green);
    color: #fff;
    font-size: 10px;
    font-weight: 700;
    font-family: 'JetBrains Mono', monospace;
    padding: 3px 8px;
    border-radius: 4px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
  }

  .quiz-title { font-weight: 600; font-size: 15px; }

  .quiz-q {
    margin-bottom: 20px;
    padding-bottom: 20px;
    border-bottom: 1px solid var(--border-subtle);
  }

  .quiz-q:last-child { border-bottom: none; margin-bottom: 0; padding-bottom: 0; }

  .quiz-question { font-size: 14px; font-weight: 500; margin-bottom: 12px; color: var(--text-primary); }

  .quiz-options { display: flex; flex-direction: column; gap: 8px; }

  .quiz-option {
    display: flex;
    align-items: flex-start;
    gap: 10px;
    padding: 10px 14px;
    border: 1px solid var(--border);
    border-radius: 6px;
    cursor: pointer;
    transition: all 0.15s;
    font-size: 13px;
    color: var(--text-secondary);
    background: var(--bg-secondary);
  }

  .quiz-option:hover { border-color: var(--spring-green); color: var(--text-primary); background: rgba(109,179,63,0.05); }
  .quiz-option.correct { border-color: var(--spring-green); background: rgba(109,179,63,0.12); color: var(--spring-light); }
  .quiz-option.wrong { border-color: var(--accent-red); background: rgba(250,121,112,0.08); color: var(--accent-red); }
  .quiz-option.disabled { pointer-events: none; }

  .quiz-dot { width: 16px; height: 16px; border-radius: 50%; border: 2px solid var(--border); flex-shrink: 0; margin-top: 2px; }
  .quiz-option.correct .quiz-dot { background: var(--spring-green); border-color: var(--spring-green); }
  .quiz-option.wrong .quiz-dot { background: var(--accent-red); border-color: var(--accent-red); }

  .quiz-explanation {
    margin-top: 10px;
    padding: 10px 14px;
    background: rgba(109,179,63,0.06);
    border-radius: 6px;
    font-size: 12.5px;
    color: var(--text-secondary);
    display: none;
    border-left: 3px solid var(--spring-green);
  }

  /* === CHALLENGE === */
  .challenge-box {
    background: linear-gradient(135deg, rgba(109,179,63,0.06), rgba(61,107,34,0.04));
    border: 1px solid rgba(109,179,63,0.3);
    border-radius: 10px;
    padding: 24px;
    margin: 32px 0;
  }

  .challenge-header {
    display: flex;
    align-items: center;
    gap: 10px;
    margin-bottom: 14px;
  }

  .challenge-icon { font-size: 20px; }
  .challenge-title { font-weight: 700; font-size: 15px; color: var(--spring-light); }
  .challenge-body { font-size: 13.5px; color: var(--text-secondary); }
  .challenge-body ul { margin: 10px 0 0 18px; }
  .challenge-body li { margin-bottom: 6px; }

  /* === LESSON NAV === */
  .lesson-nav {
    display: flex;
    justify-content: space-between;
    margin-top: 48px;
    padding-top: 24px;
    border-top: 1px solid var(--border);
  }

  .lesson-nav-btn {
    display: flex;
    flex-direction: column;
    gap: 4px;
    cursor: pointer;
    padding: 12px 16px;
    border: 1px solid var(--border);
    border-radius: 8px;
    background: var(--bg-card);
    text-decoration: none;
    transition: border-color 0.15s;
    max-width: 220px;
  }

  .lesson-nav-btn:hover { border-color: var(--spring-green); }
  .lesson-nav-label { font-size: 10px; color: var(--text-muted); font-family: 'JetBrains Mono', monospace; text-transform: uppercase; }
  .lesson-nav-title { font-size: 13px; color: var(--text-primary); font-weight: 500; }

  /* === SPRING INITIALIZR DIAGRAM === */
  .initializr-mock {
    background: #1E2936;
    border: 1px solid #2D3A4A;
    border-radius: 10px;
    overflow: hidden;
    margin: 24px 0;
  }

  .initializr-bar {
    background: #283341;
    padding: 10px 16px;
    display: flex;
    align-items: center;
    gap: 8px;
    border-bottom: 1px solid #2D3A4A;
  }

  .dot-r { width: 10px; height: 10px; border-radius: 50%; background: #FA7970; }
  .dot-y { width: 10px; height: 10px; border-radius: 50%; background: #F0883E; }
  .dot-g { width: 10px; height: 10px; border-radius: 50%; background: var(--spring-green); }

  .initializr-url {
    font-family: 'JetBrains Mono', monospace;
    font-size: 11px;
    color: var(--text-muted);
    margin-left: 8px;
  }

  .initializr-body {
    padding: 24px;
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 20px;
  }

  .init-field label {
    display: block;
    font-size: 11px;
    font-family: 'JetBrains Mono', monospace;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 6px;
  }

  .init-field .init-val {
    background: #0F1117;
    border: 1px solid #30363D;
    border-radius: 5px;
    padding: 8px 12px;
    font-family: 'JetBrains Mono', monospace;
    font-size: 12px;
    color: var(--spring-light);
  }

  .init-deps {
    grid-column: 1/-1;
  }

  .dep-chips { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 8px; }

  .dep-chip {
    background: rgba(109,179,63,0.15);
    border: 1px solid rgba(109,179,63,0.4);
    color: var(--spring-light);
    font-family: 'JetBrains Mono', monospace;
    font-size: 11px;
    padding: 4px 10px;
    border-radius: 20px;
  }

  .init-generate {
    grid-column: 1/-1;
    background: var(--spring-green);
    border: none;
    color: #fff;
    font-weight: 700;
    font-size: 13px;
    padding: 10px;
    border-radius: 6px;
    cursor: pointer;
    font-family: 'Inter', sans-serif;
    letter-spacing: 0.3px;
  }

  /* === LAYER DIAGRAM === */
  .layer-diagram {
    margin: 24px 0;
    display: flex;
    flex-direction: column;
    gap: 0;
  }

  .layer {
    padding: 14px 20px;
    border: 1px solid var(--border);
    display: flex;
    align-items: center;
    gap: 14px;
    transition: background 0.15s;
    cursor: default;
  }

  .layer:first-child { border-radius: 8px 8px 0 0; }
  .layer:last-child { border-radius: 0 0 8px 8px; border-top: none; }
  .layer:not(:first-child):not(:last-child) { border-top: none; }

  .layer:hover { background: var(--bg-elevated); }

  .layer-icon { font-size: 18px; min-width: 28px; text-align: center; }
  .layer-name { font-weight: 600; font-size: 13px; min-width: 130px; color: var(--text-primary); }
  .layer-role { font-size: 12.5px; color: var(--text-secondary); }
  .layer-ann { font-family: 'JetBrains Mono', monospace; font-size: 11px; color: var(--accent-orange); margin-left: auto; }

  .layer-controller { background: rgba(56,139,253,0.06); }
  .layer-service { background: rgba(109,179,63,0.06); }
  .layer-repo { background: rgba(188,140,255,0.06); }
  .layer-db { background: rgba(240,136,62,0.06); }

  /* === HIDDEN === */
  .lesson-panel { display: none; }
  .lesson-panel.active { display: block; }

    .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-grid, .initializr-body { grid-template-columns: 1fr !important; }
}