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

:root {
    --primary-color: #2c3e50;
    --secondary-color: #3498db;
    --success-color: #27ae60;
    --danger-color: #e74c3c;
    --warning-color: #f39c12;
    --light-bg: #ecf0f1;
    --white: #ffffff;
    --text-dark: #2c3e50;
    --text-light: #7f8c8d;
    --border-color: #bdc3c7;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    min-height: 100vh;
    padding: 20px;
    color: var(--text-dark);
}

.screen {
    display: none;
}

.screen.active {
    display: block;
}

.container {
    max-width: 800px;
    margin: 0 auto;
}

/* Welcome Screen */
#welcome-screen h1 {
    text-align: center;
    color: var(--white);
    font-size: 2.5rem;
    margin-bottom: 10px;
}

.subtitle {
    text-align: center;
    color: rgba(255, 255, 255, 0.9);
    font-size: 1.1rem;
    margin-bottom: 40px;
}

/* Stats Grid */
.stats-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 20px;
    margin-bottom: 40px;
}

.stat-card {
    background: var(--white);
    padding: 30px;
    border-radius: 12px;
    text-align: center;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
}

.stat-number {
    font-size: 2.5rem;
    font-weight: bold;
    color: var(--primary-color);
    margin-bottom: 10px;
}

.stat-label {
    color: var(--text-light);
    font-size: 0.9rem;
}

/* Buttons */
.button-group {
    display: flex;
    gap: 15px;
    margin-bottom: 30px;
    flex-wrap: wrap;
}

.btn {
    flex: 1;
    min-width: 200px;
    padding: 15px 30px;
    border: none;
    border-radius: 8px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.3s ease;
}

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

.btn-primary:hover {
    background: #2980b9;
    transform: translateY(-2px);
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
}

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

.btn-secondary:hover {
    background: #229954;
    transform: translateY(-2px);
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
}

.btn-outline {
    background: var(--white);
    color: var(--primary-color);
    border: 2px solid var(--primary-color);
}

.btn-outline:hover {
    background: var(--primary-color);
    color: var(--white);
}

.btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.btn:disabled:hover {
    transform: none;
    box-shadow: none;
}

/* Topic Selector */
.topic-selector {
    background: var(--white);
    padding: 25px;
    border-radius: 12px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
}

.topic-selector h3 {
    margin-bottom: 15px;
    color: var(--primary-color);
}

.topic-selector select {
    width: 100%;
    padding: 12px;
    border: 2px solid var(--border-color);
    border-radius: 6px;
    font-size: 1rem;
    cursor: pointer;
}

/* Quiz Screen */
.quiz-header {
    background: var(--white);
    padding: 20px;
    border-radius: 12px 12px 0 0;
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

.progress-bar {
    width: 100%;
    height: 8px;
    background: var(--light-bg);
    border-radius: 4px;
    overflow: hidden;
    margin-bottom: 15px;
}

.progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--secondary-color), var(--success-color));
    transition: width 0.3s ease;
}

.quiz-info {
    display: flex;
    justify-content: space-between;
    align-items: center;
    color: var(--text-light);
}

.timer {
    font-weight: bold;
    font-size: 1.1rem;
    color: var(--primary-color);
}

/* Question Card */
.question-card {
    background: var(--white);
    padding: 30px;
    border-radius: 0 0 12px 12px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    margin-bottom: 20px;
}

.topic-badge {
    display: inline-block;
    background: var(--secondary-color);
    color: var(--white);
    padding: 6px 12px;
    border-radius: 20px;
    font-size: 0.85rem;
    margin-bottom: 20px;
}

.question-card h2 {
    font-size: 1.4rem;
    margin-bottom: 25px;
    line-height: 1.6;
}

/* Answers */
.answers {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.answer-option {
    display: flex;
    align-items: center;
    padding: 15px;
    border: 2px solid var(--border-color);
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.2s ease;
}

.answer-option:hover {
    border-color: var(--secondary-color);
    background: rgba(52, 152, 219, 0.05);
}

.answer-option.selected {
    border-color: var(--secondary-color);
    background: rgba(52, 152, 219, 0.1);
}

.answer-option.correct {
    border-color: var(--success-color);
    background: rgba(39, 174, 96, 0.1);
}

.answer-option.incorrect {
    border-color: var(--danger-color);
    background: rgba(231, 76, 60, 0.1);
}

.answer-option input[type="radio"] {
    margin-right: 12px;
    width: 20px;
    height: 20px;
    cursor: pointer;
}

.answer-option label {
    flex: 1;
    cursor: pointer;
    font-size: 1rem;
    line-height: 1.5;
}

.explanation {
    margin-top: 20px;
    padding: 15px;
    background: var(--light-bg);
    border-left: 4px solid var(--secondary-color);
    border-radius: 4px;
    font-size: 0.95rem;
    line-height: 1.6;
}

/* Quiz Controls */
.quiz-controls {
    display: flex;
    gap: 15px;
    justify-content: space-between;
}

/* Results Screen */
#results-screen h1 {
    text-align: center;
    color: var(--white);
    font-size: 2.5rem;
    margin-bottom: 30px;
}

.results-summary {
    background: var(--white);
    padding: 40px;
    border-radius: 12px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    margin-bottom: 30px;
    text-align: center;
}

.result-circle {
    margin-bottom: 20px;
}

.result-percentage {
    font-size: 4rem;
    font-weight: bold;
    color: var(--primary-color);
}

.result-label {
    font-size: 1.2rem;
    color: var(--text-light);
    margin-top: 10px;
}

.results-details {
    font-size: 1.1rem;
    margin-top: 20px;
}

.pass-status {
    margin-top: 15px;
    padding: 12px;
    border-radius: 6px;
    font-weight: 600;
    font-size: 1.1rem;
}

.pass-status.pass {
    background: rgba(39, 174, 96, 0.1);
    color: var(--success-color);
}

.pass-status.fail {
    background: rgba(231, 76, 60, 0.1);
    color: var(--danger-color);
}

/* Topic Breakdown */
.topic-breakdown {
    background: var(--white);
    padding: 30px;
    border-radius: 12px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    margin-bottom: 30px;
}

.topic-breakdown h3 {
    margin-bottom: 20px;
    color: var(--primary-color);
}

.topic-result {
    margin-bottom: 20px;
}

.topic-result-header {
    display: flex;
    justify-content: space-between;
    margin-bottom: 8px;
    font-size: 0.95rem;
}

.topic-result-bar {
    height: 8px;
    background: var(--light-bg);
    border-radius: 4px;
    overflow: hidden;
}

.topic-result-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--secondary-color), var(--success-color));
    transition: width 0.5s ease;
}

/* Review Screen */
#review-screen h2 {
    text-align: center;
    color: var(--white);
    font-size: 2rem;
    margin-bottom: 30px;
}

.review-item {
    background: var(--white);
    padding: 25px;
    border-radius: 12px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    margin-bottom: 20px;
    border-left: 6px solid var(--border-color);
}

.review-item.correct {
    border-left-color: var(--success-color);
}

.review-item.incorrect {
    border-left-color: var(--danger-color);
}

.review-header {
    display: flex;
    gap: 15px;
    margin-bottom: 15px;
    flex-wrap: wrap;
}

.review-number {
    font-weight: bold;
    color: var(--primary-color);
}

.review-topic {
    color: var(--text-light);
    font-size: 0.9rem;
}

.review-status {
    margin-left: auto;
    font-weight: bold;
}

.review-item.correct .review-status {
    color: var(--success-color);
}

.review-item.incorrect .review-status {
    color: var(--danger-color);
}

.review-question {
    font-size: 1.1rem;
    font-weight: 600;
    margin-bottom: 15px;
    line-height: 1.5;
}

.review-answers {
    display: flex;
    flex-direction: column;
    gap: 8px;
    margin-bottom: 15px;
}

.review-answer {
    padding: 10px;
    border-radius: 6px;
    background: var(--light-bg);
}

.review-answer.correct-answer {
    background: rgba(39, 174, 96, 0.1);
    border: 2px solid var(--success-color);
    font-weight: 600;
}

.review-answer.user-answer:not(.correct-answer) {
    background: rgba(231, 76, 60, 0.1);
    border: 2px solid var(--danger-color);
}

.review-explanation {
    margin-top: 15px;
    padding: 12px;
    background: rgba(52, 152, 219, 0.05);
    border-left: 3px solid var(--secondary-color);
    border-radius: 4px;
    font-size: 0.95rem;
    line-height: 1.6;
}

/* Responsive Design */
@media (max-width: 768px) {
    body {
        padding: 10px;
    }

    #welcome-screen h1 {
        font-size: 2rem;
    }

    .subtitle {
        font-size: 1rem;
    }

    .stats-grid {
        grid-template-columns: 1fr;
    }

    .button-group {
        flex-direction: column;
    }

    .btn {
        min-width: auto;
    }

    .question-card h2 {
        font-size: 1.2rem;
    }

    .result-percentage {
        font-size: 3rem;
    }

    .quiz-controls {
        flex-direction: column;
    }
}

/* Loading animation for PWA */
@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.screen.active {
    animation: fadeIn 0.3s ease;
}