:root {
            --sage-light: #e8f5e9;
            --sage-medium: #c8e6c9;
            --beige: #f5f5dc;
            --off-white: #fafafa;
            --dark-green: #2e7d32;
            --green-accent: #4caf50;
            --amber: #ff8f00;
            --amber-light: #fff3e0;
            --formal-blue: #1976d2;
            --formal-blue-light: #e3f2fd;
            --error-red: #d32f2f;
            --success-green: #388e3c;
            --text-dark: #333;
            --text-muted: #666;
            --shadow: rgba(0,0,0,0.1);
        }

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

        body {
            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
            background: var(--off-white);
            color: var(--text-dark);
            line-height: 1.6;
        }

        /* Header */
        header {
            background: linear-gradient(135deg, var(--dark-green) 0%, #1b5e20 100%);
            color: white;
            padding: 1rem 0;
            box-shadow: 0 2px 8px var(--shadow);
            position: sticky;
            top: 0;
            z-index: 100;
        }

        .header-content {
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 1rem;
            display: flex;
            justify-content: space-between;
            align-items: center;
            flex-wrap: wrap;
            gap: 0.5rem;
        }

        .logo {
            font-size: 1.5rem;
            font-weight: 700;
            text-decoration: none;
            color: white;
            display: flex;
            align-items: center;
            gap: 0.5rem;
        }

        .logo::before {
            content: "🇩🇪";
            font-size: 1.8rem;
        }

        .exercise-indicator {
            background: rgba(255,255,255,0.2);
            padding: 0.5rem 1rem;
            border-radius: 20px;
            font-size: 0.9rem;
            font-weight: 500;
        }

        /* Navigation */
        .nav-bar {
            background: var(--sage-light);
            border-bottom: 2px solid var(--sage-medium);
            padding: 1rem 0;
            position: sticky;
            top: 64px;
            z-index: 99;
        }

        .nav-content {
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 1rem;
            display: flex;
            justify-content: space-between;
            align-items: center;
        }

        .nav-btn {
            background: white;
            border: 2px solid var(--dark-green);
            color: var(--dark-green);
            padding: 0.5rem 1.2rem;
            border-radius: 25px;
            text-decoration: none;
            font-weight: 600;
            transition: all 0.3s;
            display: inline-flex;
            align-items: center;
            gap: 0.5rem;
        }

        .nav-btn:hover {
            background: var(--dark-green);
            color: white;
            transform: translateY(-2px);
            box-shadow: 0 4px 12px var(--shadow);
        }

        .nav-btn.disabled {
            opacity: 0.5;
            pointer-events: none;
        }

        /* Main Layout */
        .container {
            max-width: 1200px;
            margin: 0 auto;
            padding: 2rem 1rem;
        }

        .section {
            background: white;
            border-radius: 12px;
            padding: 2rem;
            margin-bottom: 2rem;
            box-shadow: 0 2px 8px var(--shadow);
            border-left: 4px solid var(--dark-green);
        }

        h1 {
            color: var(--dark-green);
            font-size: 2rem;
            margin-bottom: 1.5rem;
            border-bottom: 3px solid var(--amber);
            padding-bottom: 0.5rem;
            display: inline-block;
        }

        h2 {
            color: var(--dark-green);
            font-size: 1.5rem;
            margin: 1.5rem 0 1rem;
            display: flex;
            align-items: center;
            gap: 0.5rem;
        }

        h3 {
            color: var(--text-dark);
            font-size: 1.2rem;
            margin: 1.2rem 0 0.8rem;
        }

        p {
            margin-bottom: 1rem;
            text-align: justify;
        }

        /* Theory Specific Styles */
        .theory-box {
            background: var(--sage-light);
            border-radius: 8px;
            padding: 1.5rem;
            margin: 1rem 0;
            border-left: 4px solid var(--green-accent);
        }

        .comparison-table {
            width: 100%;
            border-collapse: collapse;
            margin: 1.5rem 0;
            background: white;
            border-radius: 8px;
            overflow: hidden;
            box-shadow: 0 2px 4px var(--shadow);
        }

        .comparison-table th,
        .comparison-table td {
            padding: 1rem;
            text-align: left;
            border-bottom: 1px solid #e0e0e0;
        }

        .comparison-table th {
            background: var(--dark-green);
            color: white;
            font-weight: 600;
        }

        .comparison-table tr:hover {
            background: var(--sage-light);
        }

        .formal-cell {
            background: var(--formal-blue-light);
            color: var(--formal-blue);
            font-weight: 600;
        }

        .informal-cell {
            background: var(--sage-light);
            color: var(--dark-green);
            font-weight: 600;
        }

        .imperative-highlight {
            color: var(--amber);
            font-weight: 700;
            font-size: 1.1em;
        }

        /* Interactive Components */
        .tool-box {
            background: var(--beige);
            border-radius: 12px;
            padding: 1.5rem;
            margin: 1.5rem 0;
            border: 2px solid #e0e0e0;
        }

        .input-group {
            display: flex;
            flex-wrap: wrap;
            gap: 1rem;
            margin: 1rem 0;
            align-items: end;
        }

        input[type="text"], select {
            padding: 0.8rem;
            border: 2px solid #ddd;
            border-radius: 8px;
            font-size: 1rem;
            transition: border-color 0.3s;
            flex: 1;
            min-width: 200px;
        }

        input[type="text"]:focus, select:focus {
            outline: none;
            border-color: var(--dark-green);
            box-shadow: 0 0 0 3px rgba(46, 125, 50, 0.1);
        }

        .btn {
            background: var(--dark-green);
            color: white;
            border: none;
            padding: 0.8rem 1.5rem;
            border-radius: 8px;
            cursor: pointer;
            font-size: 1rem;
            font-weight: 600;
            transition: all 0.3s;
        }

        .btn:hover {
            background: #1b5e20;
            transform: translateY(-2px);
            box-shadow: 0 4px 12px var(--shadow);
        }

        .btn-secondary {
            background: var(--amber);
            color: white;
        }

        .btn-secondary:hover {
            background: #ef6c00;
        }

        .btn-outline {
            background: transparent;
            border: 2px solid var(--dark-green);
            color: var(--dark-green);
        }

        .btn-outline:hover {
            background: var(--sage-light);
        }

        .result-box {
            margin-top: 1rem;
            padding: 1rem;
            background: white;
            border-radius: 8px;
            border-left: 4px solid var(--amber);
            display: none;
        }

        .result-box.active {
            display: block;
            animation: slideIn 0.3s ease;
        }

        @keyframes slideIn {
            from { opacity: 0; transform: translateY(-10px); }
            to { opacity: 1; transform: translateY(0); }
        }

        .verb-form {
            display: inline-block;
            margin: 0.5rem;
            padding: 0.5rem 1rem;
            background: var(--amber-light);
            border-radius: 20px;
            font-weight: 600;
            color: var(--amber);
        }

        .separator {
            width: 100%;
            height: 2px;
            background: linear-gradient(to right, transparent, var(--sage-medium), transparent);
            margin: 2rem 0;
        }

        /* Exercise Styles */
        .exercise-container {
            background: white;
            border-radius: 12px;
            padding: 2rem;
            margin-bottom: 2rem;
            box-shadow: 0 2px 8px var(--shadow);
        }

        .exercise-header {
            display: flex;
            justify-content: space-between;
            align-items: center;
            margin-bottom: 1.5rem;
            flex-wrap: wrap;
            gap: 1rem;
        }

        .exercise-title {
            font-size: 1.3rem;
            color: var(--dark-green);
            font-weight: 700;
        }

        .exercise-item {
            display: flex;
            flex-wrap: wrap;
            align-items: center;
            gap: 0.5rem;
            margin: 1rem 0;
            padding: 1rem;
            background: var(--off-white);
            border-radius: 8px;
            border-left: 3px solid var(--sage-medium);
        }

        .exercise-number {
            font-weight: 700;
            color: var(--dark-green);
            min-width: 30px;
        }

        .exercise-input {
            flex: 1;
            min-width: 250px;
            padding: 0.6rem;
            border: 2px solid #ddd;
            border-radius: 6px;
            font-size: 1rem;
        }

        .exercise-input.correct {
            border-color: var(--success-green);
            background: #e8f5e9;
        }

        .exercise-input.incorrect {
            border-color: var(--error-red);
            background: #ffebee;
        }

        .hint {
            font-size: 0.9rem;
            color: var(--text-muted);
            font-style: italic;
            margin-left: 2rem;
            width: 100%;
        }

        .feedback {
            margin-left: 1rem;
            font-weight: 600;
            font-size: 0.9rem;
            display: none;
        }

        .feedback.show {
            display: inline;
        }

        .feedback.correct {
            color: var(--success-green);
        }

        .feedback.incorrect {
            color: var(--error-red);
        }

        .solution {
            display: none;
            color: var(--dark-green);
            font-weight: 600;
            margin-left: 1rem;
            font-size: 0.9rem;
        }

        .solution.show {
            display: inline;
        }

        .action-buttons {
            display: flex;
            gap: 1rem;
            margin-top: 1.5rem;
            flex-wrap: wrap;
        }

        /* Visual Indicators */
        .politeness-indicator {
            display: flex;
            gap: 1rem;
            margin: 1rem 0;
            flex-wrap: wrap;
        }

        .indicator-box {
            flex: 1;
            min-width: 200px;
            padding: 1rem;
            border-radius: 8px;
            text-align: center;
            cursor: pointer;
            transition: all 0.3s;
            border: 2px solid transparent;
        }

        .indicator-box:hover {
            transform: translateY(-2px);
            box-shadow: 0 4px 12px var(--shadow);
        }

        .indicator-box.active {
            border-color: var(--dark-green);
            box-shadow: 0 0 0 3px rgba(46, 125, 50, 0.2);
        }

        .indicator-informal {
            background: var(--sage-light);
            color: var(--dark-green);
        }

        .indicator-formal {
            background: var(--formal-blue-light);
            color: var(--formal-blue);
        }

        .toggle-switch {
            display: inline-flex;
            align-items: center;
            gap: 0.5rem;
            margin: 0.5rem 0;
            cursor: pointer;
        }

        .toggle-switch input {
            width: 50px;
            height: 25px;
            appearance: none;
            background: #ccc;
            border-radius: 25px;
            position: relative;
            cursor: pointer;
            transition: background 0.3s;
        }

        .toggle-switch input::after {
            content: '';
            position: absolute;
            width: 20px;
            height: 20px;
            background: white;
            border-radius: 50%;
            top: 2.5px;
            left: 2.5px;
            transition: transform 0.3s;
            box-shadow: 0 2px 4px rgba(0,0,0,0.2);
        }

        .toggle-switch input:checked {
            background: var(--dark-green);
        }

        .toggle-switch input:checked::after {
            transform: translateX(25px);
        }

        /* Context Selector */
        .context-selector {
            background: var(--beige);
            padding: 1.5rem;
            border-radius: 12px;
            margin: 1.5rem 0;
        }

        .context-buttons {
            display: flex;
            gap: 0.5rem;
            margin-top: 0.5rem;
            flex-wrap: wrap;
        }

        .context-btn {
            padding: 0.5rem 1rem;
            border: 2px solid var(--dark-green);
            background: white;
            color: var(--dark-green);
            border-radius: 20px;
            cursor: pointer;
            font-weight: 600;
            transition: all 0.3s;
        }

        .context-btn.active {
            background: var(--dark-green);
            color: white;
        }

        .context-recommendation {
            margin-top: 1rem;
            padding: 1rem;
            background: white;
            border-radius: 8px;
            border-left: 4px solid var(--amber);
            display: none;
        }

        .context-recommendation.active {
            display: block;
        }

        /* Progress Bar */
        .progress-container {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            height: 4px;
            background: rgba(255,255,255,0.3);
            z-index: 101;
        }

        .progress-bar {
            height: 100%;
            background: var(--amber);
            width: 0%;
            transition: width 0.3s;
        }

        /* Checklist */
        .checklist {
            background: var(--sage-light);
            padding: 1.5rem;
            border-radius: 8px;
            margin: 1rem 0;
        }

        .checklist-item {
            display: flex;
            align-items: start;
            gap: 0.5rem;
            margin: 0.8rem 0;
        }

        .checklist-item::before {
            content: "☐";
            color: var(--dark-green);
            font-weight: bold;
            font-size: 1.2rem;
        }

        .checklist-item.checked::before {
            content: "☑";
        }

        /* Animation for separable verbs */
        .separable-demo {
            font-size: 1.5rem;
            text-align: center;
            padding: 2rem;
            background: white;
            border-radius: 8px;
            margin: 1rem 0;
            font-family: monospace;
        }

        .prefix {
            display: inline-block;
            transition: all 0.5s;
            color: var(--amber);
            font-weight: bold;
        }

        .stem {
            display: inline-block;
            color: var(--dark-green);
            font-weight: bold;
        }

        .separated .prefix {
            transform: translateX(100px);
            color: var(--error-red);
        }

        .arrow {
            opacity: 0;
            transition: opacity 0.3s;
            color: var(--text-muted);
        }

        .separated .arrow {
            opacity: 1;
        }

        /* Footer */
        footer {
            background: var(--dark-green);
            color: white;
            padding: 2rem 0;
            margin-top: 3rem;
        }

        .footer-content {
            max-width: 1200px;
            margin: 0 auto;
            padding: 0 1rem;
            display: flex;
            justify-content: space-between;
            align-items: center;
            flex-wrap: wrap;
            gap: 1rem;
        }

        .footer-link {
            color: white;
            text-decoration: none;
            padding: 0.5rem 1rem;
            border: 2px solid rgba(255,255,255,0.3);
            border-radius: 20px;
            transition: all 0.3s;
        }

        .footer-link:hover {
            background: rgba(255,255,255,0.1);
            border-color: white;
        }

        .completion-checkbox {
            display: flex;
            align-items: center;
            gap: 0.5rem;
            background: rgba(255,255,255,0.1);
            padding: 0.8rem 1.5rem;
            border-radius: 25px;
            cursor: pointer;
        }

        .completion-checkbox input {
            width: 20px;
            height: 20px;
            cursor: pointer;
        }

        /* Responsive */
        @media (max-width: 768px) {
            .header-content {
                flex-direction: column;
                text-align: center;
            }

            .nav-content {
                flex-direction: column;
                gap: 1rem;
            }

            .exercise-header {
                flex-direction: column;
                align-items: flex-start;
            }

            .action-buttons {
                flex-direction: column;
                width: 100%;
            }

            .btn {
                width: 100%;
                text-align: center;
            }

            .comparison-table {
                font-size: 0.9rem;
            }

            .comparison-table th,
            .comparison-table td {
                padding: 0.5rem;
            }
        }

        /* Accessibility */
        .sr-only {
            position: absolute;
            width: 1px;
            height: 1px;
            padding: 0;
            margin: -1px;
            overflow: hidden;
            clip: rect(0,0,0,0);
            border: 0;
        }

        *:focus {
            outline: 2px solid var(--amber);
            outline-offset: 2px;
        }

        /* Print styles */
        @media print {
            .nav-bar, .action-buttons, .tool-box, .progress-container {
                display: none;
            }

            .section {
                break-inside: avoid;
                box-shadow: none;
                border: 1px solid #ddd;
            }
        }