:root {
            /* Color scheme */
            --primary: #4f46e5;
            --primary-dark: #3730a3;
            --secondary: #06b6d4;
            --accent: #f59e0b;
            --success: #10b981;
            --error: #ef4444;
            --warning: #f59e0b;
            --bg: #f8fafc;
            --card-bg: #ffffff;
            --text: #1e293b;
            --text-light: #64748b;
            --border: #e2e8f0;
            
            /* Modal verb colors */
            --durfen: #8b5cf6;
            --konnen: #3b82f6;
            --mussen: #ef4444;
            --sollen: #10b981;
            --wollen: #f59e0b;
            --mogen: #ec4899;
            --mochten: #06b6d4;
        }
        
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        
        body {
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
            background: var(--bg);
            color: var(--text);
            line-height: 1.6;
            padding-bottom: 2rem;
        }
        
        /* Header */
        .chapter-header {
            background: linear-gradient(135deg, var(--primary) 0%, var(--primary-dark) 100%);
            color: white;
            padding: 2rem 1rem;
            text-align: center;
        }
        
        .chapter-badge {
            display: inline-block;
            background: rgba(255,255,255,0.2);
            padding: 0.5rem 1rem;
            border-radius: 2rem;
            font-size: 0.875rem;
            font-weight: 600;
            margin-bottom: 1rem;
            text-transform: uppercase;
            letter-spacing: 0.05em;
        }
        
        .chapter-header h1 {
            font-size: 2rem;
            margin-bottom: 0.5rem;
        }
        
        .chapter-header p {
            opacity: 0.9;
            max-width: 600px;
            margin: 0 auto;
        }
        
        /* Navigation */
        .chapter-nav {
            display: flex;
            justify-content: space-between;
            align-items: center;
            padding: 1rem;
            background: var(--card-bg);
            border-bottom: 1px solid var(--border);
            flex-wrap: wrap;
            gap: 0.5rem;
        }
        
        .chapter-nav a {
            color: var(--primary);
            text-decoration: none;
            padding: 0.5rem 1rem;
            border-radius: 0.5rem;
            transition: all 0.2s;
            font-weight: 500;
        }
        
        .chapter-nav a:hover {
            background: #eef2ff;
        }
        
        .progress-bar {
            font-size: 0.875rem;
            color: var(--text-light);
            font-weight: 500;
        }
        
        /* Container */
        .container {
            max-width: 900px;
            margin: 0 auto;
            padding: 1rem;
        }
        
        /* Cards */
        .card {
            background: var(--card-bg);
            border-radius: 1rem;
            padding: 1.5rem;
            margin-bottom: 1.5rem;
            box-shadow: 0 1px 3px rgba(0,0,0,0.1);
        }
        
        .card h2 {
            color: var(--primary);
            margin-bottom: 1rem;
            font-size: 1.5rem;
        }
        
        .card h3 {
            color: var(--text);
            margin: 1.5rem 0 0.75rem;
            font-size: 1.25rem;
        }
        
        .card h4 {
            color: var(--text);
            margin: 1rem 0 0.5rem;
            font-size: 1.1rem;
        }
        
        /* Modal verb cards */
        .modal-card {
            border-left: 4px solid;
            margin-bottom: 1.5rem;
        }
        
        .modal-card.durfen { border-color: var(--durfen); }
        .modal-card.konnen { border-color: var(--konnen); }
        .modal-card.mussen { border-color: var(--mussen); }
        .modal-card.sollen { border-color: var(--sollen); }
        .modal-card.wollen { border-color: var(--wollen); }
        .modal-card.mogen { border-color: var(--mogen); }
        .modal-card.mochten { border-color: var(--mochten); }
        
        .modal-header {
            display: flex;
            align-items: center;
            gap: 1rem;
            margin-bottom: 1rem;
        }
        
        .modal-title {
            font-size: 1.5rem;
            font-weight: 700;
        }
        
        .modal-title.durfen { color: var(--durfen); }
        .modal-title.konnen { color: var(--konnen); }
        .modal-title.mussen { color: var(--mussen); }
        .modal-title.sollen { color: var(--sollen); }
        .modal-title.wollen { color: var(--wollen); }
        .modal-title.mogen { color: var(--mogen); }
        .modal-title.mochten { color: var(--mochten); }
        
        .modal-meaning {
            background: #f1f5f9;
            padding: 0.25rem 0.75rem;
            border-radius: 1rem;
            font-size: 0.875rem;
            color: var(--text-light);
        }
        
        /* Conjugation tables */
        .conj-table {
            width: 100%;
            border-collapse: collapse;
            margin: 1rem 0;
            font-size: 0.95rem;
        }
        
        .conj-table th,
        .conj-table td {
            padding: 0.75rem;
            text-align: left;
            border-bottom: 1px solid var(--border);
        }
        
        .conj-table th {
            background: #f8fafc;
            font-weight: 600;
            color: var(--text-light);
        }
        
        .conj-table tr:hover {
            background: #f8fafc;
        }
        
        .irregular {
            font-style: italic;
            font-weight: 600;
        }
        
        .durfen .irregular { color: var(--durfen); }
        .konnen .irregular { color: var(--konnen); }
        .mussen .irregular { color: var(--mussen); }
        .wollen .irregular { color: var(--wollen); }
        .mogen .irregular { color: var(--mogen); }
        
        /* Example sentences */
        .example {
            background: #f8fafc;
            padding: 1rem;
            border-radius: 0.5rem;
            margin: 0.75rem 0;
            display: flex;
            align-items: flex-start;
            gap: 0.75rem;
        }
        
        .example-text {
            flex: 1;
        }
        
        .example-german {
            font-weight: 500;
            margin-bottom: 0.25rem;
        }
        
        .example-english {
            color: var(--text-light);
            font-size: 0.9rem;
        }
        
        /* Audio button */
        .audio-btn {
            background: var(--primary);
            color: white;
            border: none;
            width: 32px;
            height: 32px;
            border-radius: 50%;
            cursor: pointer;
            display: flex;
            align-items: center;
            justify-content: center;
            transition: all 0.2s;
            flex-shrink: 0;
        }
        
        .audio-btn:hover {
            background: var(--primary-dark);
            transform: scale(1.1);
        }
        
        .audio-btn:active {
            transform: scale(0.95);
        }
        
        /* Word order diagram */
        .word-order {
            background: linear-gradient(135deg, #eef2ff 0%, #e0e7ff 100%);
            padding: 1.5rem;
            border-radius: 0.75rem;
            margin: 1rem 0;
        }
        
        .sentence-structure {
            display: flex;
            flex-wrap: wrap;
            gap: 0.5rem;
            align-items: center;
            justify-content: center;
            margin: 1rem 0;
        }
        
        .word-box {
            padding: 0.5rem 1rem;
            border-radius: 0.5rem;
            font-weight: 600;
        }
        
        .word-box.subject { background: #dbeafe; color: #1e40af; }
        .word-box.modal { background: #fecaca; color: #991b1b; }
        .word-box.other { background: #d1fae5; color: #065f46; }
        .word-box.verb { background: #fef3c7; color: #92400e; }
        
        .position-label {
            text-align: center;
            font-size: 0.75rem;
            color: var(--text-light);
            margin-top: 0.25rem;
        }
        
        /* Summary table */
        .summary-table {
            width: 100%;
            border-collapse: collapse;
            margin: 1rem 0;
            font-size: 0.9rem;
        }
        
        .summary-table th,
        .summary-table td {
            padding: 0.6rem;
            text-align: center;
            border: 1px solid var(--border);
        }
        
        .summary-table th {
            background: var(--primary);
            color: white;
            font-weight: 600;
        }
        
        .summary-table td:first-child {
            font-weight: 600;
            background: #f8fafc;
        }
        
        .summary-table .irregular {
            font-style: italic;
            font-weight: 700;
        }
        
        /* Exercises */
        .exercise-section {
            background: var(--card-bg);
            border-radius: 1rem;
            padding: 1.5rem;
            margin-bottom: 1.5rem;
            box-shadow: 0 1px 3px rgba(0,0,0,0.1);
        }
        
        .exercise-section h2 {
            color: var(--primary);
            margin-bottom: 0.5rem;
        }
        
        .exercise-instructions {
            color: var(--text-light);
            margin-bottom: 1.5rem;
            font-style: italic;
        }
        
        .exercise-item {
            margin-bottom: 1.5rem;
            padding: 1rem;
            background: #f8fafc;
            border-radius: 0.5rem;
        }
        
        .exercise-number {
            display: inline-flex;
            align-items: center;
            justify-content: center;
            width: 28px;
            height: 28px;
            background: var(--primary);
            color: white;
            border-radius: 50%;
            font-size: 0.875rem;
            font-weight: 600;
            margin-right: 0.5rem;
        }
        
        .exercise-prompt {
            margin-bottom: 0.75rem;
        }
        
        /* Input styles */
        .input-group {
            display: flex;
            flex-wrap: wrap;
            gap: 0.5rem;
            align-items: center;
        }
        
        .verb-input {
            padding: 0.5rem 0.75rem;
            border: 2px solid var(--border);
            border-radius: 0.375rem;
            font-size: 1rem;
            min-width: 100px;
            transition: all 0.2s;
        }
        
        .verb-input:focus {
            outline: none;
            border-color: var(--primary);
        }
        
        .verb-input.correct {
            border-color: var(--success);
            background: #ecfdf5;
        }
        
        .verb-input.incorrect {
            border-color: var(--error);
            background: #fef2f2;
        }
        
        .check-btn {
            background: var(--primary);
            color: white;
            border: none;
            padding: 0.5rem 1rem;
            border-radius: 0.375rem;
            cursor: pointer;
            font-weight: 500;
            transition: all 0.2s;
        }
        
        .check-btn:hover {
            background: var(--primary-dark);
        }
        
        .feedback {
            margin-top: 0.5rem;
            padding: 0.5rem;
            border-radius: 0.375rem;
            font-size: 0.9rem;
            display: none;
        }
        
        .feedback.show {
            display: block;
        }
        
        .feedback.correct {
            background: #ecfdf5;
            color: #065f46;
        }
        
        .feedback.incorrect {
            background: #fef2f2;
            color: #991b1b;
        }
        
        /* Conjugation exercise table */
        .conj-exercise-table {
            width: 100%;
            border-collapse: collapse;
            margin: 1rem 0;
        }
        
        .conj-exercise-table th,
        .conj-exercise-table td {
            padding: 0.5rem;
            border: 1px solid var(--border);
            text-align: center;
        }
        
        .conj-exercise-table th {
            background: #f8fafc;
            font-weight: 600;
        }
        
        .conj-exercise-table input {
            width: 100%;
            padding: 0.4rem;
            border: 2px solid var(--border);
            border-radius: 0.25rem;
            text-align: center;
            font-size: 0.9rem;
        }
        
        /* Translation exercise */
        .translation-item {
            display: flex;
            flex-direction: column;
            gap: 0.5rem;
            margin-bottom: 1rem;
            padding: 1rem;
            background: #f8fafc;
            border-radius: 0.5rem;
        }
        
        .translation-english {
            font-weight: 500;
        }
        
        .translation-forms {
            display: flex;
            flex-direction: column;
            gap: 0.5rem;
        }
        
        .translation-form {
            display: flex;
            align-items: center;
            gap: 0.5rem;
        }
        
        .form-label {
            font-size: 0.8rem;
            color: var(--text-light);
            width: 40px;
        }
        
        /* Checklist */
        .checklist-item {
            display: flex;
            align-items: flex-start;
            gap: 0.75rem;
            padding: 1rem;
            background: #f8fafc;
            border-radius: 0.5rem;
            margin-bottom: 0.75rem;
            cursor: pointer;
            transition: all 0.2s;
        }
        
        .checklist-item:hover {
            background: #f1f5f9;
        }
        
        .checklist-checkbox {
            width: 20px;
            height: 20px;
            border: 2px solid var(--border);
            border-radius: 0.25rem;
            display: flex;
            align-items: center;
            justify-content: center;
            flex-shrink: 0;
            margin-top: 2px;
        }
        
        .checklist-item.checked .checklist-checkbox {
            background: var(--success);
            border-color: var(--success);
        }
        
        .checklist-item.checked .checklist-checkbox::after {
            content: '✓';
            color: white;
            font-size: 0.75rem;
        }
        
        .checklist-answer {
            display: none;
            margin-top: 0.5rem;
            padding: 0.75rem;
            background: #ecfdf5;
            border-radius: 0.375rem;
            color: #065f46;
        }
        
        .checklist-item.show-answer .checklist-answer {
            display: block;
        }
        
        /* Bridge section */
        .connectivity {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 1rem;
            margin-top: 2rem;
        }
        
        .from-prev, .to-next {
            padding: 1.5rem;
            border-radius: 0.75rem;
        }
        
        .from-prev {
            background: linear-gradient(135deg, #fef3c7 0%, #fde68a 100%);
        }
        
        .to-next {
            background: linear-gradient(135deg, #dbeafe 0%, #bfdbfe 100%);
        }
        
        .from-prev h4, .to-next h4 {
            margin-bottom: 0.5rem;
            font-size: 1rem;
        }
        
        .from-prev h4 { color: #92400e; }
        .to-next h4 { color: #1e40af; }
        
        .from-prev p, .to-next p {
            font-size: 0.9rem;
            color: var(--text);
        }
        
        /* Streak counter */
        .streak-counter {
            position: fixed;
            bottom: 1rem;
            right: 1rem;
            background: var(--card-bg);
            padding: 0.75rem 1rem;
            border-radius: 2rem;
            box-shadow: 0 4px 12px rgba(0,0,0,0.15);
            display: flex;
            align-items: center;
            gap: 0.5rem;
            font-weight: 600;
            z-index: 100;
        }
        
        .streak-icon {
            font-size: 1.25rem;
        }
        
        /* Info box */
        .info-box {
            background: #fef3c7;
            border-left: 4px solid var(--accent);
            padding: 1rem;
            border-radius: 0 0.5rem 0.5rem 0;
            margin: 1rem 0;
        }
        
        .info-box h4 {
            color: #92400e;
            margin-bottom: 0.5rem;
        }
        
        .info-box p {
            color: #78350f;
            font-size: 0.95rem;
        }
        
        /* Note box */
        .note-box {
            background: #dbeafe;
            border-left: 4px solid var(--konnen);
            padding: 1rem;
            border-radius: 0 0.5rem 0.5rem 0;
            margin: 1rem 0;
        }
        
        .note-box strong {
            color: #1e40af;
        }
        
        /* Responsive */
        @media (max-width: 768px) {
            .chapter-header h1 {
                font-size: 1.5rem;
            }
            
            .chapter-nav {
                flex-direction: column;
                text-align: center;
            }
            
            .modal-header {
                flex-direction: column;
                align-items: flex-start;
            }
            
            .summary-table {
                font-size: 0.75rem;
            }
            
            .summary-table th,
            .summary-table td {
                padding: 0.4rem;
            }
            
            .connectivity {
                grid-template-columns: 1fr;
            }
            
            .conj-exercise-table {
                font-size: 0.8rem;
            }
            
            .conj-exercise-table input {
                padding: 0.3rem;
            }
        }
        
        /* Utility */
        .highlight {
            background: #fef3c7;
            padding: 0.125rem 0.375rem;
            border-radius: 0.25rem;
        }
        
        .verb-highlight {
            font-weight: 600;
        }
        
        .verb-highlight.durfen { color: var(--durfen); }
        .verb-highlight.konnen { color: var(--konnen); }
        .verb-highlight.mussen { color: var(--mussen); }
        .verb-highlight.sollen { color: var(--sollen); }
        .verb-highlight.wollen { color: var(--wollen); }
        .verb-highlight.mogen { color: var(--mogen); }
        .verb-highlight.mochten { color: var(--mochten); }