:root {
    --primary-color-light: #0ea5e9; --secondary-color-light: #475569; --background-color-light: #f8fafc;
    --text-color-light: #334155; --header-color-light: #0c4a6e; --sidebar-bg-light: #f1f5f9;
    --sidebar-link-light: #475569; --sidebar-link-hover-light: #0369a1; --sidebar-active-bg-light: #e0f2fe;
    --card-bg-light: #ffffff; --border-color-light: #e2e8f0;

    --primary-color-dark: #38bdf8; --secondary-color-dark: #94a3b8; --background-color-dark: #0f172a;
    --text-color-dark: #cbd5e1; --header-color-dark: #bae6fd; --sidebar-bg-dark: #1e293b;
    --sidebar-link-dark: #94a3b8; --sidebar-link-hover-dark: #e0f2fe; --sidebar-active-bg-dark: #334155;
    --card-bg-dark: #1e293b; --border-color-dark: #334155;
    
    --shadow: 0 4px 6px -1px rgba(0,0,0,0.1), 0 2px 4px -2px rgba(0,0,0,0.1);
    --font-size: 16px;
  }
  body {
    --primary-color: var(--primary-color-light); --secondary-color: var(--secondary-color-light); --background-color: var(--background-color-light);
    --text-color: var(--text-color-light); --header-color: var(--header-color-light); --sidebar-bg: var(--sidebar-bg-light);
    --sidebar-link: var(--sidebar-link-light); --sidebar-link-hover: var(--sidebar-link-hover-light); --sidebar-active-bg: var(--sidebar-active-bg-light);
    --card-bg: var(--card-bg-light); --border-color: var(--border-color-light);
  }
  body.dark-theme {
    --primary-color: var(--primary-color-dark); --secondary-color: var(--secondary-color-dark); --background-color: var(--background-color-dark);
    --text-color: var(--text-color-dark); --header-color: var(--header-color-dark); --sidebar-bg: var(--sidebar-bg-dark);
    --sidebar-link: var(--sidebar-link-dark); --sidebar-link-hover: var(--sidebar-link-hover-dark); --sidebar-active-bg: var(--sidebar-active-bg-dark);
    --card-bg: var(--card-bg-dark); --border-color: var(--border-color-dark);
  }
  *, *::before, *::after { box-sizing: border-box; }
  body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    line-height: 1.6; color: var(--text-color); background-color: var(--background-color);
    margin: 0; display: flex; height: 100vh; overflow: hidden; font-size: var(--font-size); transition: background-color 0.3s, color 0.3s;
  }
  .sidebar {
    width: 320px; background-color: var(--sidebar-bg); border-right: 1px solid var(--border-color);
    display: flex; flex-direction: column; transition: transform 0.3s ease, background-color 0.3s, border-color 0.3s;
  }
  .sidebar-header { padding: 1.5rem; border-bottom: 1px solid var(--border-color); }
  .sidebar-header h2 { font-size: 1.25rem; color: var(--header-color); margin: 0; }
  .sidebar-progress { padding: 1.5rem; }
  .progress-bar { width: 100%; height: 8px; background-color: var(--border-color); border-radius: 4px; overflow: hidden; }
  .progress-bar-inner { height: 100%; width: 0%; background-color: var(--primary-color); transition: width 0.3s ease; border-radius: 4px; }
  .progress-text { font-size: 0.875rem; color: var(--secondary-color); text-align: center; margin-top: 0.5rem; }
  .sidebar-nav { flex-grow: 1; overflow-y: auto; padding: 1rem; }
  .sidebar-nav ul { list-style: none; padding: 0; margin: 0; }
  .chapter-summary { cursor: pointer; padding: 0.75rem 1rem; font-weight: 600; color: var(--header-color); display: flex; justify-content: space-between; align-items: center; border-radius: 6px; }
  .chapter-summary:hover { background-color: var(--border-color); }
  .chapter-details[open] > .chapter-summary::after { transform: rotate(180deg); }
  .lesson-list { padding-left: 1rem; }
  .lesson-item button {
    width: 100%; text-align: left; background: none; border: none; padding: 0.6rem 1rem; font-size: 0.9rem;
    color: var(--sidebar-link); border-radius: 6px; cursor: pointer; display: flex; align-items: center; gap: 0.75rem;
    transition: background-color 0.2s, color 0.2s;
  }
  .lesson-item button:hover { background-color: var(--sidebar-active-bg); color: var(--sidebar-link-hover); }
  .lesson-item.active button { background-color: var(--sidebar-active-bg); color: var(--sidebar-link-hover); font-weight: 500; }
  .lesson-status { width: 1rem; height: 1rem; border: 2px solid #94a3b8; border-radius: 50%; transition: all 0.2s; }
  .lesson-item.completed .lesson-status { background-color: var(--primary-color); border-color: var(--primary-color); color: white; display: flex; align-items: center; justify-content: center; font-size: 0.7rem; }
  .lesson-item.completed .lesson-status::before { content: '✔'; }

  main { flex-grow: 1; padding: 2rem 3rem; overflow-y: auto; position: relative; }
  .lesson-content { opacity: 0; transform: translateY(10px); transition: opacity 0.4s ease, transform 0.4s ease; }
  .lesson-content.visible { opacity: 1; transform: translateY(0); }
  .lesson-header h1 { font-size: 2.5rem; color: var(--header-color); margin: 0 0 0.5rem 0; }
  .lesson-header p { font-style: italic; color: var(--secondary-color); }
  .lesson-summary { font-style: italic; color: var(--primary-color); background-color: var(--sidebar-active-bg); padding: 1rem; border-left: 4px solid var(--primary-color); border-radius: 4px; margin-bottom: 1.5rem; }
  .lesson-body { margin-top: 2rem; }
  .lesson-image { max-width: 100%; height: auto; border-radius: 8px; box-shadow: var(--shadow); margin: 2rem 0; }
  .interactive-zone { background-color: var(--card-bg); border: 1px solid var(--border-color); padding: 1.5rem; margin-top: 2rem; border-radius: 8px; }
  .interactive-zone h4 { margin-top: 0; }
  .quiz-option { display: block; margin: 0.5rem 0; }
  .quiz-feedback { padding: 0.5rem 1rem; border-radius: 6px; margin-top: 1rem; }
  .quiz-feedback.correct { background-color: #dcfce7; color: #166534; }
  .quiz-feedback.incorrect { background-color: #fee2e2; color: #991b1b; }
  body.dark-theme .quiz-feedback.correct { background-color: #14532d; color: #bbf7d0; }
  body.dark-theme .quiz-feedback.incorrect { background-color: #7f1d1d; color: #fecaca; }
  
  .lesson-footer { margin-top: 3rem; padding-top: 1.5rem; border-top: 1px solid var(--border-color); }
  .completion-zone { display: flex; justify-content: space-between; align-items: center; }
  .rating-stars span { font-size: 1.5rem; cursor: pointer; color: #cbd5e1; transition: color 0.2s; }
  .rating-stars span:hover, .rating-stars span.active { color: #f59e0b; }
  .navigation-buttons { display: flex; justify-content: space-between; align-items: center; margin-top: 2rem; }
  .btn {
    background-color: var(--primary-color); color: white; border: none; padding: 0.75rem 1.5rem; border-radius: 6px;
    font-size: 1rem; font-weight: 500; cursor: pointer; transition: background-color 0.2s;
  }
  .btn:hover { opacity: 0.8; }
  .btn:disabled { background-color: #94a3b8; cursor: not-allowed; }
  .menu-toggle { display: none; }
  
  .export-controls {
    position: fixed; top: 10px; right: 10px; z-index: 9999; background: var(--card-bg);
    border: 1px solid var(--border-color); border-radius: 8px; padding: 8px;
    display: flex; gap: 8px; align-items: center; box-shadow: var(--shadow);
  }
  .control-btn {
    background: var(--sidebar-bg); border: 1px solid var(--border-color);
    color: var(--text-color); padding: 4px 8px; border-radius: 4px;
    cursor: pointer; font-size: 12px;
  }
  .control-btn:hover { background-color: var(--sidebar-active-bg); }

  @media (max-width: 768px) {
    body { flex-direction: column; }
    .sidebar { width: 100%; height: auto; max-height: 50vh; transform: none; }
    .menu-toggle { display: block; position: fixed; top: 1rem; left: 1rem; z-index: 1000; background: var(--card-bg); border-radius: 50%; padding: 0.5rem; border: 1px solid var(--border-color); }
    main { padding: 2rem 1rem; }
  }