/* Base Typography and Colors inspired by Anthropic's minimal elegance */
:root {
    --bg-color: #f5f4ef; /* Warm off-white */
    --text-primary: #2d2c2b; /* Soft black/dark brown */
    --text-secondary: #6e6b66;
    --accent-color: #d97757; /* Muted terracotta */
    --border-color: #e2e0d8;
    --paper-bg: #ffffff;
    
    /* Functional colors */
    --success-bg: #eaf5eb;
    --success-text: #2a7e3b;
    --error-bg: #fbeded;
    --error-text: #b33a3a;
    
    /* Typography */
    --font-serif: 'Crimson Pro', serif;
    --font-sans: 'Inter', system-ui, sans-serif;
    
    /* Spacing */
    --space-sm: 0.5rem;
    --space-md: 1rem;
    --space-lg: 2rem;
    --space-xl: 3rem;
}

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

body {
    background-color: var(--bg-color);
    color: var(--text-primary);
    font-family: var(--font-sans);
    line-height: 1.6;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

h1, h2, h3, h4, h5, h6 {
    font-family: var(--font-serif);
    font-weight: 600;
    line-height: 1.2;
}

/* Layout */
.layout {
    display: flex;
    min-height: 100vh;
    max-width: 1400px;
    margin: 0 auto;
}

/* Sidebar */
.sidebar {
    width: 320px;
    flex-shrink: 0;
    border-right: 1px solid var(--border-color);
    padding: var(--space-xl) var(--space-lg);
    background-color: var(--paper-bg);
    position: sticky;
    top: 0;
    height: 100vh;
    overflow-y: auto;
}

.logo {
    margin-bottom: var(--space-xl);
}

.logo h1 {
    font-size: 2rem;
    letter-spacing: -0.02em;
}

.filter-group {
    margin-bottom: var(--space-lg);
}

.filter-group label {
    display: block;
    font-size: 0.85rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-secondary);
    margin-bottom: var(--space-sm);
    font-weight: 600;
}

select {
    width: 100%;
    padding: 0.75rem;
    font-family: var(--font-sans);
    font-size: 1rem;
    border: 1px solid var(--border-color);
    border-radius: 6px;
    background-color: transparent;
    color: var(--text-primary);
    appearance: none;
    background-image: url("data:image/svg+xml;charset=US-ASCII,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22292.4%22%20height%3D%22292.4%22%3E%3Cpath%20fill%3D%22%232d2c2b%22%20d%3D%22M287%2069.4a17.6%2017.6%200%200%200-13-5.4H18.4c-5%200-9.3%201.8-12.9%205.4A17.6%2017.6%200%200%200%200%2082.2c0%205%201.8%209.3%205.4%2012.9l128%20127.9c3.6%203.6%207.8%205.4%2012.8%205.4s9.2-1.8%2012.8-5.4L287%2095c3.5-3.5%205.4-7.8%205.4-12.8%200-5-1.9-9.2-5.5-12.8z%22%2F%3E%3C%2Fsvg%3E");
    background-repeat: no-repeat;
    background-position: right 1rem top 50%;
    background-size: 0.65em auto;
    cursor: pointer;
    transition: border-color 0.2s ease;
}

select:focus {
    outline: none;
    border-color: var(--accent-color);
}

select:disabled {
    color: var(--text-secondary);
    opacity: 0.7;
    cursor: not-allowed;
}

/* Chapter List */
.chapter-list {
    list-style: none;
}

.chapter-item {
    padding: 0.6rem 0.75rem;
    margin-bottom: 0.25rem;
    border-radius: 6px;
    cursor: pointer;
    transition: all 0.2s ease;
    font-size: 0.95rem;
    color: var(--text-secondary);
}

.chapter-item:hover {
    background-color: var(--bg-color);
    color: var(--text-primary);
}

.chapter-item.active {
    background-color: var(--bg-color);
    color: var(--text-primary);
    font-weight: 500;
}

/* Main Content */
.main-content {
    flex-grow: 1;
    padding: var(--space-xl);
    max-width: 800px;
    margin: 0 auto;
}

.content-header {
    margin-bottom: var(--space-xl);
}

.content-header h2 {
    font-size: 2.5rem;
    margin-bottom: var(--space-sm);
}

.content-header p {
    color: var(--text-secondary);
    font-size: 1.1rem;
}

/* Progress */
.progress-container {
    height: 4px;
    background-color: var(--border-color);
    border-radius: 2px;
    margin-top: var(--space-md);
    overflow: hidden;
}

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

/* Empty State */
.empty-state {
    text-align: center;
    padding: var(--space-xl) 0;
    color: var(--text-secondary);
}

.empty-state svg {
    margin-bottom: var(--space-md);
    opacity: 0.5;
}

/* MCQ Card */
.question-card {
    background-color: var(--paper-bg);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: var(--space-lg);
    margin-bottom: var(--space-lg);
    box-shadow: 0 4px 20px rgba(0,0,0,0.02);
    animation: fadeIn 0.4s ease-out forwards;
}

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

.question-text {
    font-family: var(--font-serif);
    font-size: 1.5rem;
    margin-bottom: var(--space-lg);
    color: var(--text-primary);
}

.options-list {
    display: flex;
    flex-direction: column;
    gap: var(--space-md);
}

.option-btn {
    text-align: left;
    padding: 1rem;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    background-color: transparent;
    font-family: var(--font-sans);
    font-size: 1rem;
    color: var(--text-primary);
    cursor: pointer;
    transition: all 0.2s ease;
    display: flex;
    align-items: center;
}

.option-btn:hover:not(:disabled) {
    border-color: #c4c1b5;
    background-color: #faf9f5;
}

.option-letter {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 28px;
    height: 28px;
    border-radius: 4px;
    background-color: var(--bg-color);
    margin-right: 1rem;
    font-weight: 600;
    font-size: 0.85rem;
    color: var(--text-secondary);
}

/* States */
.option-btn.correct {
    background-color: var(--success-bg);
    border-color: #b9debf;
    color: var(--success-text);
}

.option-btn.correct .option-letter {
    background-color: #fff;
    color: var(--success-text);
}

.option-btn.wrong {
    background-color: var(--error-bg);
    border-color: #f2c7c7;
    color: var(--error-text);
}

.option-btn.wrong .option-letter {
    background-color: #fff;
    color: var(--error-text);
}

.option-btn:disabled {
    cursor: default;
}

/* Explanation */
.explanation {
    margin-top: var(--space-lg);
    padding-top: var(--space-md);
    border-top: 1px dashed var(--border-color);
    animation: fadeIn 0.4s ease-out forwards;
}

.explanation-badge {
    display: inline-block;
    padding: 0.25rem 0.6rem;
    border-radius: 4px;
    font-size: 0.75rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    margin-bottom: var(--space-sm);
}

.explanation-badge.correct {
    background-color: var(--success-bg);
    color: var(--success-text);
}

.explanation-badge.wrong {
    background-color: var(--error-bg);
    color: var(--error-text);
}

.explanation-text {
    color: var(--text-secondary);
    font-size: 0.95rem;
    line-height: 1.7;
}

.next-btn-container {
    margin-top: var(--space-md);
    text-align: right;
}

.primary-btn {
    background-color: var(--text-primary);
    color: #fff;
    border: none;
    padding: 0.75rem 1.5rem;
    border-radius: 6px;
    font-family: var(--font-sans);
    font-size: 0.95rem;
    font-weight: 500;
    cursor: pointer;
    transition: background-color 0.2s ease;
}

.primary-btn:hover {
    background-color: #1a1a1a;
}

.secondary-btn {
    background-color: transparent;
    color: var(--text-primary);
    border: 1px solid var(--border-color);
    padding: 0.75rem 1.5rem;
    border-radius: 6px;
    font-family: var(--font-sans);
    font-size: 0.95rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
}

.secondary-btn:hover {
    border-color: var(--text-primary);
}

.quiz-completion {
    text-align: center;
    padding: var(--space-xl);
    background-color: var(--paper-bg);
    border-radius: 12px;
    border: 1px solid var(--border-color);
    margin-top: var(--space-lg);
}

.quiz-completion h3 {
    font-size: 1.8rem;
    margin-bottom: var(--space-sm);
}

.quiz-completion p {
    color: var(--text-secondary);
    margin-bottom: var(--space-lg);
}

/* Responsive */
@media (max-width: 900px) {
    .layout {
        flex-direction: column;
    }
    .sidebar {
        width: 100%;
        height: auto;
        border-right: none;
        border-bottom: 1px solid var(--border-color);
        position: relative;
        padding: var(--space-md);
    }
    .main-content {
        padding: var(--space-md);
    }
}