/* Reset và Base Styles */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --primary-color: #4a90e2;
    --secondary-color: #50c878;
    --danger-color: #e74c3c;
    --warning-color: #f39c12;
    --text-color: #2c3e50;
    --bg-color: #f5f7fa;
    --card-bg: #ffffff;
    --border-color: #e1e8ed;
    --shadow: 0 2px 8px rgba(0,0,0,0.1);
    --shadow-hover: 0 4px 12px rgba(0,0,0,0.15);
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    line-height: 1.6;
    min-height: 100vh;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 20px;
}

/* Header Styles */
.header {
    text-align: center;
    padding: 40px 20px;
    background: linear-gradient(135deg, var(--primary-color), var(--secondary-color));
    color: white;
    border-radius: 16px;
    margin-bottom: 40px;
    box-shadow: var(--shadow);
}

.header h1 {
    font-size: 2.5rem;
    margin-bottom: 10px;
}

.subtitle {
    font-size: 1.1rem;
    opacity: 0.95;
}

/* Subjects Container */
.subjects-container {
    display: grid;
    gap: 30px;
}

.subject-card {
    background: var(--card-bg);
    border-radius: 12px;
    padding: 30px;
    box-shadow: var(--shadow);
    transition: transform 0.2s, box-shadow 0.2s;
}

.subject-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-hover);
}

.subject-header {
    display: flex;
    align-items: center;
    gap: 12px;
    margin-bottom: 20px;
    padding-bottom: 15px;
    border-bottom: 2px solid var(--border-color);
}

.subject-icon {
    font-size: 2rem;
}

.subject-header h2 {
    font-size: 1.8rem;
    color: var(--primary-color);
}

.grades-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 15px;
}

.grade-card {
    background: linear-gradient(135deg, var(--primary-color), #5ba3f5);
    color: white;
    padding: 20px;
    border-radius: 10px;
    text-align: center;
    cursor: pointer;
    transition: transform 0.2s, box-shadow 0.2s;
    text-decoration: none;
    display: block;
}

.grade-card:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-hover);
}

.grade-number {
    font-size: 2.5rem;
    font-weight: bold;
    margin-bottom: 5px;
}

.grade-label {
    font-size: 1rem;
    opacity: 0.95;
}

.question-count {
    font-size: 0.9rem;
    margin-top: 10px;
    opacity: 0.9;
}

/* Quiz Header */
.quiz-header {
    background: var(--card-bg);
    padding: 20px;
    border-radius: 12px;
    margin-bottom: 30px;
    box-shadow: var(--shadow);
}

.quiz-info {
    display: flex;
    align-items: center;
    gap: 20px;
    margin-bottom: 20px;
    flex-wrap: wrap;
}

.back-btn {
    padding: 10px 20px;
    background: var(--border-color);
    color: var(--text-color);
    text-decoration: none;
    border-radius: 8px;
    transition: background 0.2s;
}

.back-btn:hover {
    background: #d1d8dd;
}

.progress-bar {
    width: 100%;
}

.progress-text {
    display: flex;
    justify-content: space-between;
    margin-bottom: 8px;
    font-weight: 600;
}

.progress-track {
    width: 100%;
    height: 10px;
    background: var(--border-color);
    border-radius: 10px;
    overflow: hidden;
}

.progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--primary-color), var(--secondary-color));
    border-radius: 10px;
    transition: width 0.3s ease;
    width: 0%;
}

/* Question Container */
.question-container {
    background: var(--card-bg);
    padding: 30px;
    border-radius: 12px;
    margin-bottom: 30px;
    box-shadow: var(--shadow);
    min-height: 300px;
}

.question-text {
    font-size: 1.3rem;
    font-weight: 600;
    margin-bottom: 30px;
    line-height: 1.8;
}

.answers-list {
    display: flex;
    flex-direction: column;
    gap: 15px;
}

.answer-option {
    padding: 18px 20px;
    background: var(--bg-color);
    border: 2px solid var(--border-color);
    border-radius: 10px;
    cursor: pointer;
    transition: all 0.2s;
    display: flex;
    align-items: center;
    gap: 15px;
}

.answer-option:hover {
    border-color: var(--primary-color);
    background: #f0f7ff;
}

.answer-option.selected {
    border-color: var(--primary-color);
    background: #e6f2ff;
    font-weight: 600;
}

.answer-option.correct {
    border-color: var(--secondary-color);
    background: #e8f8f0;
}

.answer-option.incorrect {
    border-color: var(--danger-color);
    background: #fee;
}

.answer-key {
    font-weight: bold;
    color: var(--primary-color);
    min-width: 30px;
}

.answer-option.correct .answer-key {
    color: var(--secondary-color);
}

.answer-option.incorrect .answer-key {
    color: var(--danger-color);
}

/* Navigation Buttons */
.navigation-buttons {
    display: flex;
    gap: 15px;
    justify-content: center;
    flex-wrap: wrap;
    margin-bottom: 30px;
}

.btn {
    padding: 12px 30px;
    border: none;
    border-radius: 8px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s;
    text-decoration: none;
    display: inline-block;
    text-align: center;
}

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

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

.btn-primary:hover:not(:disabled) {
    background: #3a7bc8;
}

.btn-secondary {
    background: #95a5a6;
    color: white;
}

.btn-secondary:hover:not(:disabled) {
    background: #7f8c8d;
}

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

.btn-success:hover {
    background: #45b369;
}

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

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

/* Question Grid */
.question-grid-container {
    margin-top: 30px;
    text-align: center;
}

.question-grid-container h3 {
    font-size: 1.2rem;
    margin-bottom: 15px;
}

.question-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(50px, 1fr));
    gap: 10px;
    padding: 20px;
    background: var(--card-bg);
    border-radius: 12px;
    box-shadow: var(--shadow);
}

.question-number-btn {
    padding: 12px;
    background: var(--bg-color);
    border: 2px solid var(--border-color);
    border-radius: 8px;
    cursor: pointer;
    font-weight: 600;
    transition: all 0.2s;
}

.question-number-btn:hover {
    border-color: var(--primary-color);
}

.question-number-btn.active {
    background: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
}

.question-number-btn.answered {
    background: #e6f2ff;
    border-color: var(--primary-color);
}

/* Modal */
.modal {
    display: none;
    position: fixed;
    z-index: 1000;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.5);
    align-items: center;
    justify-content: center;
    padding: 20px;
}

.modal.show {
    display: flex;
}

.modal-content {
    background: var(--card-bg);
    padding: 40px;
    border-radius: 16px;
    max-width: 600px;
    width: 100%;
    max-height: 90vh;
    overflow-y: auto;
    box-shadow: 0 10px 40px rgba(0,0,0,0.3);
}

.modal-content h2 {
    color: var(--primary-color);
    margin-bottom: 20px;
    text-align: center;
}

.result-summary {
    text-align: center;
    padding: 30px;
    background: var(--bg-color);
    border-radius: 12px;
    margin-bottom: 20px;
}

.score-display {
    font-size: 3rem;
    font-weight: bold;
    color: var(--secondary-color);
    margin: 10px 0;
}

.result-details {
    margin: 20px 0;
}

.result-item {
    display: flex;
    justify-content: space-between;
    padding: 12px 0;
    border-bottom: 1px solid var(--border-color);
}

.modal-buttons {
    display: flex;
    gap: 10px;
    justify-content: center;
    flex-wrap: wrap;
    margin-top: 20px;
}

/* Footer */
.footer {
    text-align: center;
    padding: 20px;
    margin-top: 40px;
    color: #7f8c8d;
}

/* Loading */
.loading {
    text-align: center;
    padding: 40px;
    font-size: 1.2rem;
    color: #7f8c8d;
}

/* Responsive Design */
@media (max-width: 768px) {
    .header h1 {
        font-size: 1.8rem;
    }

    .container {
        padding: 10px;
    }

    .question-text {
        font-size: 1.1rem;
    }

    .grades-grid {
        grid-template-columns: repeat(2, 1fr);
    }

    .navigation-buttons {
        flex-direction: column;
    }

    .btn {
        width: 100%;
    }

    .quiz-info {
        flex-direction: column;
        align-items: flex-start;
    }

    .modal-content {
        padding: 20px;
    }

    .score-display {
        font-size: 2rem;
    }
}

@media (max-width: 480px) {
    .header h1 {
        font-size: 1.5rem;
    }

    .subtitle {
        font-size: 0.9rem;
    }

    .question-container {
        padding: 20px;
    }

    .question-grid {
        grid-template-columns: repeat(auto-fill, minmax(40px, 1fr));
    }
}