/* ==========================================
   CSS Variables & Root Styles
   ========================================== */
   :root {
    /* Color Palette */
    --primary-color: #667eea;
    --primary-dark: #5568d3;
    --primary-light: #7c8ff0;
    --secondary-color: #764ba2;
    --accent-color: #f093fb;
    
    /* Gradient Colors */
    --gradient-primary: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --gradient-card: linear-gradient(135deg, rgba(102, 126, 234, 0.1) 0%, rgba(118, 75, 162, 0.1) 100%);
    --gradient-hover: linear-gradient(135deg, rgba(102, 126, 234, 0.2) 0%, rgba(118, 75, 162, 0.2) 100%);
    
    /* Background Colors */
    --bg-primary: #0a0e27;
    --bg-secondary: #151932;
    --bg-card: #1a1f3a;
    --bg-card-hover: #1f2544;
    
    /* Text Colors */
    --text-primary: #ffffff;
    --text-secondary: #a0aec0;
    --text-muted: #718096;
    
    /* Border & Shadow */
    --border-color: rgba(255, 255, 255, 0.1);
    --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.1);
    --shadow-md: 0 4px 16px rgba(0, 0, 0, 0.2);
    --shadow-lg: 0 8px 32px rgba(0, 0, 0, 0.3);
    --shadow-glow: 0 0 40px rgba(102, 126, 234, 0.3);
    
    /* Spacing */
    --spacing-xs: 0.5rem;
    --spacing-sm: 1rem;
    --spacing-md: 1.5rem;
    --spacing-lg: 2rem;
    --spacing-xl: 3rem;
    --spacing-2xl: 4rem;
    
    /* Border Radius */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --radius-xl: 24px;
    
    /* Transitions */
    --transition-fast: 0.2s ease;
    --transition-base: 0.3s ease;
    --transition-slow: 0.5s ease;
    
    /* Typography */
    --font-primary: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    --font-weight-normal: 400;
    --font-weight-medium: 500;
    --font-weight-semibold: 600;
    --font-weight-bold: 700;
  }
  
  /* ==========================================
     Global Reset & Base Styles
     ========================================== */
  * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  html {
    scroll-behavior: smooth;
    font-size: 16px;
  }
  
  body {
    font-family: var(--font-primary);
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
    overflow-x: hidden;
  }
  
  /* ==========================================
     Skip Link for Accessibility
     ========================================== */
  .skip-link {
    position: absolute;
    top: -100px;
    left: 0;
    background: var(--primary-color);
    color: white;
    padding: var(--spacing-sm) var(--spacing-md);
    text-decoration: none;
    border-radius: 0 0 var(--radius-sm) 0;
    font-weight: var(--font-weight-semibold);
    z-index: 1000;
    transition: top var(--transition-fast);
  }
  
  .skip-link:focus {
    top: 0;
  }
  
  /* ==========================================
     Utility Classes
     ========================================== */
  .container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 var(--spacing-md);
  }
  
  .visually-hidden {
    position: absolute;
    width: 1px;
    height: 1px;
    margin: -1px;
    padding: 0;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    white-space: nowrap;
    border: 0;
  }
  
  /* ==========================================
     Header Styles
     ========================================== */
  .ebook-header {
    background: var(--gradient-primary);
    padding: var(--spacing-xl) 0;
    text-align: center;
    position: relative;
    overflow: hidden;
  }
  
  .ebook-header::before {
    content: '';
    position: absolute;
    top: -50%;
    right: -10%;
    width: 500px;
    height: 500px;
    background: radial-gradient(circle, rgba(255, 255, 255, 0.1) 0%, transparent 70%);
    animation: float 20s ease-in-out infinite;
  }
  
  .ebook-header::after {
    content: '';
    position: absolute;
    bottom: -50%;
    left: -10%;
    width: 400px;
    height: 400px;
    background: radial-gradient(circle, rgba(255, 255, 255, 0.08) 0%, transparent 70%);
    animation: float 15s ease-in-out infinite reverse;
  }
  
  @keyframes float {
    0%, 100% {
      transform: translate(0, 0) rotate(0deg);
    }
    33% {
      transform: translate(30px, -30px) rotate(120deg);
    }
    66% {
      transform: translate(-20px, 20px) rotate(240deg);
    }
  }
  
  .ebook-header .container {
    position: relative;
    z-index: 1;
  }
  
  .ebook-header h1 {
    font-size: clamp(2rem, 5vw, 3.5rem);
    font-weight: var(--font-weight-bold);
    margin-bottom: var(--spacing-sm);
    display: flex;
    align-items: center;
    justify-content: center;
    gap: var(--spacing-sm);
    text-shadow: 0 2px 20px rgba(0, 0, 0, 0.3);
  }
  
  .course-badge {
    font-size: 2.5rem;
    animation: bounce 2s ease-in-out infinite;
  }
  
  @keyframes bounce {
    0%, 100% {
      transform: translateY(0);
    }
    50% {
      transform: translateY(-10px);
    }
  }
  
  .subtitle {
    font-size: clamp(1rem, 2vw, 1.25rem);
    color: rgba(255, 255, 255, 0.9);
    margin-bottom: var(--spacing-lg);
    font-weight: var(--font-weight-medium);
  }
  
  /* Progress Bar */
  .progress-container {
    max-width: 600px;
    margin: var(--spacing-lg) auto 0;
    background: rgba(255, 255, 255, 0.2);
    border-radius: var(--radius-xl);
    padding: 4px;
    backdrop-filter: blur(10px);
  }
  
  .progress-bar {
    background: linear-gradient(90deg, rgba(255, 255, 255, 0.9) 0%, rgba(255, 255, 255, 0.7) 100%);
    height: 32px;
    border-radius: var(--radius-lg);
    display: flex;
    align-items: center;
    justify-content: center;
    width: 0%;
    transition: width 0.6s cubic-bezier(0.65, 0, 0.35, 1);
    box-shadow: 0 2px 10px rgba(0, 0, 0, 0.2);
  }
  
  .progress-text {
    font-size: 0.875rem;
    font-weight: var(--font-weight-semibold);
    color: var(--primary-dark);
    white-space: nowrap;
    opacity: 0;
    transition: opacity var(--transition-base);
  }
  
  .progress-bar[aria-valuenow]:not([aria-valuenow="0"]) .progress-text {
    opacity: 1;
  }
  
  /* ==========================================
     Session Intro
     ========================================== */
  .session-intro {
    background: var(--bg-secondary);
    padding: var(--spacing-xl) 0;
    text-align: center;
    border-bottom: 1px solid var(--border-color);
  }
  
  .session-heading {
    font-size: clamp(1.75rem, 4vw, 2.5rem);
    font-weight: var(--font-weight-bold);
    margin-bottom: var(--spacing-sm);
    background: var(--gradient-primary);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
  }
  
  .session-description {
    font-size: clamp(1rem, 2vw, 1.125rem);
    color: var(--text-secondary);
    max-width: 700px;
    margin: 0 auto;
  }
  
  /* ==========================================
     Main Content Area
     ========================================== */
  main {
    padding: var(--spacing-2xl) 0;
    min-height: calc(100vh - 400px);
  }
  
  /* ==========================================
     Lesson Groups
     ========================================== */
  .lesson-group {
    margin-bottom: var(--spacing-2xl);
  }
  
  .lesson-group:last-child {
    margin-bottom: 0;
  }
  
  .group-title {
    font-size: clamp(1.5rem, 3vw, 2rem);
    font-weight: var(--font-weight-bold);
    margin-bottom: var(--spacing-lg);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    color: var(--text-primary);
    padding: 0 var(--spacing-md);
  }
  
  .group-icon {
    font-size: 1.75rem;
    filter: drop-shadow(0 2px 4px rgba(0, 0, 0, 0.3));
  }
  
  /* ==========================================
     Cards Grid
     ========================================== */
  .cards-wrap {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
    gap: var(--spacing-lg);
    padding: 0 var(--spacing-md);
    margin-bottom: var(--spacing-lg);
  }
  
  /* ==========================================
     Individual Cards
     ========================================== */
  .card {
    position: relative;
    display: block;
    text-decoration: none;
    color: inherit;
    height: 100%;
    border-radius: var(--radius-lg);
    overflow: hidden;
    transition: transform var(--transition-base), box-shadow var(--transition-base);
  }
  
  .card:hover {
    transform: translateY(-8px);
    box-shadow: var(--shadow-glow);
  }
  
  .card:focus-within {
    outline: 3px solid var(--primary-color);
    outline-offset: 4px;
  }
  
  /* Card Glow Effect */
  .card-glow {
    position: absolute;
    inset: 0;
    background: var(--gradient-card);
    opacity: 0;
    transition: opacity var(--transition-base);
    pointer-events: none;
    z-index: 1;
  }
  
  .card:hover .card-glow {
    opacity: 1;
  }
  
  /* Card Inner Content */
  .card-inner {
    position: relative;
    z-index: 2;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: var(--spacing-lg);
    height: 100%;
    display: flex;
    flex-direction: column;
    transition: background var(--transition-base), border-color var(--transition-base);
  }
  
  .card:hover .card-inner {
    background: var(--bg-card-hover);
    border-color: rgba(102, 126, 234, 0.3);
  }
  
  /* Card Top Section */
  .card-top {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: var(--spacing-md);
  }
  
  .card-icon {
    font-size: 2.5rem;
    filter: drop-shadow(0 2px 8px rgba(0, 0, 0, 0.3));
    transition: transform var(--transition-base);
  }
  
  .card:hover .card-icon {
    transform: scale(1.1) rotate(5deg);
  }
  
  .lesson-number {
    background: var(--gradient-primary);
    color: white;
    font-size: 0.875rem;
    font-weight: var(--font-weight-bold);
    padding: 0.375rem 0.75rem;
    border-radius: var(--radius-sm);
    box-shadow: var(--shadow-sm);
  }
  
  /* Card Title */
  .card-title {
    font-size: clamp(1.125rem, 2vw, 1.375rem);
    font-weight: var(--font-weight-semibold);
    margin-bottom: var(--spacing-sm);
    line-height: 1.3;
    color: var(--text-primary);
    transition: color var(--transition-fast);
  }
  
  .card:hover .card-title {
    color: var(--primary-light);
  }
  
  /* Card Text */
  .card-text {
    color: var(--text-secondary);
    font-size: 0.9375rem;
    line-height: 1.6;
    flex-grow: 1;
    margin-bottom: var(--spacing-md);
  }
  
  /* Card Footer */
  .card-footer {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding-top: var(--spacing-sm);
    border-top: 1px solid var(--border-color);
    margin-top: auto;
  }
  
  .card-cta {
    color: var(--primary-light);
    font-weight: var(--font-weight-semibold);
    font-size: 0.9375rem;
    display: inline-flex;
    align-items: center;
    gap: 0.375rem;
    transition: gap var(--transition-fast), color var(--transition-fast);
  }
  
  .card:hover .card-cta {
    gap: 0.625rem;
    color: var(--accent-color);
  }
  
  /* Difficulty Badges */
  .card-meta {
    display: flex;
    gap: var(--spacing-xs);
  }
  
  .difficulty {
    font-size: 0.75rem;
    font-weight: var(--font-weight-semibold);
    padding: 0.25rem 0.625rem;
    border-radius: var(--radius-sm);
    text-transform: uppercase;
    letter-spacing: 0.5px;
  }
  
  .difficulty.beginner {
    background: rgba(72, 187, 120, 0.2);
    color: #68d391;
    border: 1px solid rgba(72, 187, 120, 0.3);
  }
  
  .difficulty.intermediate {
    background: rgba(237, 137, 54, 0.2);
    color: #f6ad55;
    border: 1px solid rgba(237, 137, 54, 0.3);
  }
  
  .difficulty.advanced {
    background: rgba(245, 101, 101, 0.2);
    color: #fc8181;
    border: 1px solid rgba(245, 101, 101, 0.3);
  }
  
  /* Completed State */
  .card[data-completed="true"] .card-inner {
    background: linear-gradient(135deg, rgba(72, 187, 120, 0.1) 0%, rgba(56, 161, 105, 0.1) 100%);
    border-color: rgba(72, 187, 120, 0.3);
  }
  
  .card[data-completed="true"] .card-inner::after {
    content: '✓';
    position: absolute;
    top: var(--spacing-md);
    right: var(--spacing-md);
    background: #48bb78;
    color: white;
    width: 32px;
    height: 32px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: var(--font-weight-bold);
    font-size: 1.125rem;
    box-shadow: 0 2px 8px rgba(72, 187, 120, 0.4);
  }
  
  /* ==========================================
     Footer Styles
     ========================================== */
  .ebook-footer {
    background: var(--bg-secondary);
    padding: var(--spacing-xl) 0;
    border-top: 1px solid var(--border-color);
    text-align: center;
  }
  
  .ebook-footer .container {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: var(--spacing-md);
  }
  
  .back-button {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-xs);
    text-decoration: none;
    color: var(--primary-light);
    font-weight: var(--font-weight-semibold);
    font-size: 1.0625rem;
    padding: var(--spacing-sm) var(--spacing-lg);
    border: 2px solid var(--primary-color);
    border-radius: var(--radius-md);
    background: transparent;
    transition: all var(--transition-base);
    position: relative;
    overflow: hidden;
  }
  
  .back-button::before {
    content: '';
    position: absolute;
    inset: 0;
    background: var(--gradient-primary);
    opacity: 0;
    transition: opacity var(--transition-base);
    z-index: -1;
  }
  
  .back-button:hover,
  .back-button:focus {
    color: white;
    border-color: var(--primary-light);
    transform: translateY(-2px);
    box-shadow: 0 4px 16px rgba(102, 126, 234, 0.4);
  }
  
  .back-button:hover::before,
  .back-button:focus::before {
    opacity: 1;
  }
  
  .footer-info {
    margin-top: var(--spacing-sm);
  }
  
  .footer-text {
    color: var(--text-muted);
    font-size: 0.9375rem;
  }
  
  /* ==========================================
     Responsive Design
     ========================================== */
  @media (max-width: 768px) {
    .cards-wrap {
      grid-template-columns: 1fr;
      gap: var(--spacing-md);
    }
    
    .ebook-header {
      padding: var(--spacing-lg) 0;
    }
    
    main {
      padding: var(--spacing-lg) 0;
    }
    
    .lesson-group {
      margin-bottom: var(--spacing-xl);
    }
    
    .group-title {
      font-size: 1.5rem;
      padding: 0 var(--spacing-sm);
    }
    
    .cards-wrap {
      padding: 0 var(--spacing-sm);
    }
    
    .session-intro {
      padding: var(--spacing-lg) 0;
    }
  }
  
  @media (max-width: 480px) {
    :root {
      font-size: 14px;
    }
    
    .card-inner {
      padding: var(--spacing-md);
    }
    
    .card-icon {
      font-size: 2rem;
    }
    
    .lesson-number {
      font-size: 0.75rem;
      padding: 0.25rem 0.5rem;
    }
  }
  
  /* ==========================================
     Print Styles
     ========================================== */
  @media print {
    body {
      background: white;
      color: black;
    }
    
    .ebook-header,
    .ebook-footer,
    .card-glow,
    .card-cta {
      display: none;
    }
    
    .card {
      page-break-inside: avoid;
      border: 1px solid #ccc;
      box-shadow: none;
    }
  }
  
  /* ==========================================
     Animations & Keyframes
     ========================================== */
  @keyframes fadeInUp {
    from {
      opacity: 0;
      transform: translateY(30px);
    }
    to {
      opacity: 1;
      transform: translateY(0);
    }
  }
  
  .card {
    animation: fadeInUp 0.6s ease backwards;
  }
  
  .card:nth-child(1) { animation-delay: 0.1s; }
  .card:nth-child(2) { animation-delay: 0.2s; }
  .card:nth-child(3) { animation-delay: 0.3s; }
  .card:nth-child(4) { animation-delay: 0.1s; }
  .card:nth-child(5) { animation-delay: 0.2s; }
  .card:nth-child(6) { animation-delay: 0.3s; }
  
  /* ==========================================
     Reduced Motion
     ========================================== */
  @media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
      animation-duration: 0.01ms !important;
      animation-iteration-count: 1 !important;
      transition-duration: 0.01ms !important;
      scroll-behavior: auto !important;
    }
  }