:root {
    --primary-color: #1a237e;
    --secondary-color: #3949ab;
    --accent-color: #ff6f00;
    --text-dark: #212121;
    --text-light: #757575;
    --bg-light: #f5f5f5;
    --bg-white: #ffffff;
    --border-color: #e0e0e0;
    --success: #4caf50;
    --warning: #ff9800;
}

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

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

/* Header */
.header {
    background: linear-gradient(135deg, var(--primary-color) 0%, var(--secondary-color) 100%);
    color: white;
    padding: 2rem 0;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

.header h1 {
    font-size: 2.5rem;
    margin-bottom: 0.5rem;
}

.header .tagline {
    font-size: 1.1rem;
    opacity: 0.9;
}

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

/* Navigation */
.nav {
    background: var(--bg-white);
    border-bottom: 1px solid var(--border-color);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav ul {
    list-style: none;
    display: flex;
    padding: 1rem 0;
    overflow-x: auto;
}

.nav li {
    margin-right: 2rem;
}

.nav a {
    color: var(--text-dark);
    text-decoration: none;
    font-weight: 500;
    transition: color 0.3s;
}

.nav a:hover, .nav a.active {
    color: var(--accent-color);
}

/* Main Content */
.main-content {
    background: var(--bg-white);
    margin: 2rem 0;
    padding: 2rem;
    border-radius: 8px;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
}

/* Lesson Cards */
.lesson-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 2rem;
    margin-top: 2rem;
}

.lesson-card {
    background: var(--bg-white);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 1.5rem;
    transition: all 0.3s;
    text-decoration: none;
    color: inherit;
    display: block;
}

.lesson-card:hover {
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(0,0,0,0.1);
    border-color: var(--accent-color);
}

.lesson-number {
    display: inline-block;
    background: var(--accent-color);
    color: white;
    padding: 0.25rem 0.75rem;
    border-radius: 20px;
    font-size: 0.9rem;
    margin-bottom: 1rem;
}

.lesson-card h3 {
    color: var(--primary-color);
    margin-bottom: 0.5rem;
}

.lesson-card p {
    color: var(--text-light);
    line-height: 1.5;
}

/* Lesson Content */
.lesson-content {
    max-width: 900px;
    margin: 0 auto;
}

.lesson-content h1 {
    color: var(--primary-color);
    margin-bottom: 2rem;
    padding-bottom: 1rem;
    border-bottom: 2px solid var(--border-color);
}

.lesson-content h2 {
    color: var(--secondary-color);
    margin: 2rem 0 1rem;
}

.lesson-content h3 {
    color: var(--text-dark);
    margin: 1.5rem 0 1rem;
}

.lesson-content p {
    margin-bottom: 1rem;
    line-height: 1.8;
}

.lesson-content ul, .lesson-content ol {
    margin: 1rem 0 1rem 2rem;
}

.lesson-content li {
    margin-bottom: 0.5rem;
}

/* Math Blocks */
.math-block {
    background: var(--bg-light);
    border-left: 4px solid var(--accent-color);
    padding: 1.5rem;
    margin: 2rem 0;
    border-radius: 4px;
    overflow-x: auto;
}

.equation-label {
    color: var(--text-light);
    font-size: 0.9rem;
    margin-top: 0.5rem;
    font-style: italic;
}

/* Important Notes */
.important-note {
    background: #fff3e0;
    border-left: 4px solid var(--warning);
    padding: 1rem;
    margin: 1.5rem 0;
    border-radius: 4px;
}

.important-note strong {
    color: var(--warning);
}

/* Key Concept */
.key-concept {
    background: #e8f5e9;
    border-left: 4px solid var(--success);
    padding: 1rem;
    margin: 1.5rem 0;
    border-radius: 4px;
}

.key-concept strong {
    color: var(--success);
}

/* Code Blocks */
.code-block {
    background: #263238;
    color: #aed581;
    padding: 1rem;
    border-radius: 4px;
    overflow-x: auto;
    margin: 1rem 0;
    font-family: 'Courier New', monospace;
}

/* Tables */
table {
    width: 100%;
    border-collapse: collapse;
    margin: 1.5rem 0;
}

th {
    background: var(--primary-color);
    color: white;
    padding: 0.75rem;
    text-align: left;
}

td {
    padding: 0.75rem;
    border-bottom: 1px solid var(--border-color);
}

tr:hover {
    background: var(--bg-light);
}

/* Navigation Buttons */
.lesson-nav {
    display: flex;
    justify-content: space-between;
    margin-top: 3rem;
    padding-top: 2rem;
    border-top: 1px solid var(--border-color);
}

.lesson-nav a {
    display: inline-block;
    padding: 0.75rem 1.5rem;
    background: var(--secondary-color);
    color: white;
    text-decoration: none;
    border-radius: 4px;
    transition: background 0.3s;
}

.lesson-nav a:hover {
    background: var(--primary-color);
}

.lesson-nav a.disabled {
    background: var(--border-color);
    cursor: not-allowed;
    opacity: 0.5;
}

/* Footer */
.footer {
    background: var(--text-dark);
    color: white;
    padding: 2rem 0;
    margin-top: 4rem;
    text-align: center;
}

.footer a {
    color: var(--accent-color);
    text-decoration: none;
}

.footer a:hover {
    text-decoration: underline;
}

/* Responsive */
@media (max-width: 768px) {
    .header h1 {
        font-size: 2rem;
    }
    
    .lesson-grid {
        grid-template-columns: 1fr;
    }
    
    .nav ul {
        flex-direction: column;
    }
    
    .nav li {
        margin-right: 0;
        margin-bottom: 0.5rem;
    }
    
    .lesson-nav {
        flex-direction: column;
        gap: 1rem;
    }
    
    .lesson-nav a {
        text-align: center;
    }
}

/* MathJax overrides */
.MathJax {
    font-size: 1.1em !important;
}

.MathJax_Display {
    margin: 1rem 0 !important;
}