:root {
    --primary-color: #2c3e50;
    --accent-blue: #3498db;
    --accent-green: #2ecc71;
    --accent-gold: #f39c12;
    --bg-light: #f8f9fa;
    --text-dark: #2c3e50;
    --card-shadow: 0 4px 12px rgba(0,0,0,0.08);
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: var(--bg-light);
    color: var(--text-dark);
    margin: 0;
}

/* Header & Nav */
header {
    background: linear-gradient(135deg, #1a2a6c, #b21f1f, #fdbb2d); /* Vibrant Gradient */
    color: white;
    padding: 2rem 5%;
}

.header-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.course-badge {
    background: rgba(255,255,255,0.2);
    padding: 10px 20px;
    border-radius: 30px;
    font-weight: bold;
    border: 1px solid rgba(255,255,255,0.4);
}

nav {
    background: var(--primary-color);
    display: flex;
    padding: 0 5%;
}

nav a {
    color: white;
    text-decoration: none;
    padding: 15px 25px;
    transition: 0.3s;
}

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

/* Layout Container */
.container {
    display: grid;
    grid-template-columns: 280px 1fr;
    gap: 30px;
    padding: 40px 5%;
    max-width: 1400px;
    margin: auto;
}

/* Sidebar Styling */
.info-card {
    background: white;
    padding: 20px;
    border-radius: 12px;
    box-shadow: var(--card-shadow);
    margin-bottom: 20px;
}

.progress-bar-bg {
    background: #eee;
    height: 10px;
    border-radius: 5px;
    margin: 15px 0;
}

.progress-bar-fill {
    background: var(--accent-green);
    height: 100%;
    border-radius: 5px;
}

.deadline-card {
    border-left: 5px solid var(--accent-gold);
}

/* Lecture Grid (3 per row) */
.lecture-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr); /* THREE LECTURES PER ROW */
    gap: 20px;
}

.lecture-box {
    background: white;
    text-decoration: none;
    color: inherit;
    border-radius: 12px;
    padding: 20px;
    border: 1px solid #eee;
    transition: transform 0.2s, box-shadow 0.2s;
    display: flex;
    flex-direction: column;
    box-shadow: var(--card-shadow);
}

.lecture-box:hover {
    transform: translateY(-5px);
    box-shadow: 0 8px 20px rgba(0,0,0,0.12);
}

.lec-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 15px;
}

.lec-number {
    font-size: 1.5rem;
    font-weight: 800;
    opacity: 0.3;
}

.status-icon {
    font-size: 1.2rem;
}

/* Status Color Variations */
.lecture-box.completed {
    border-top: 4px solid var(--accent-green);
}
.lecture-box.completed .status-icon { color: var(--accent-green); }

.lecture-box.current {
    border-top: 4px solid var(--accent-blue);
    background: #f0f7ff;
}
.lecture-box.current .status-icon { color: var(--accent-blue); }

.lecture-box.locked {
    opacity: 0.6;
    background: #fdfdfd;
    cursor: not-allowed;
    border-top: 4px solid #ccc;
}

/* Responsive adjustments */
@media (max-width: 1100px) {
    .lecture-grid { grid-template-columns: repeat(2, 1fr); }
}

@media (max-width: 768px) {
    .container { grid-template-columns: 1fr; }
    .lecture-grid { grid-template-columns: 1fr; }
}