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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: #f4f7f6;
    color: #333;
    line-height: 1.6;
}

/* Header Styling */
header {
    background-color: #4a90e2; /* Friendly blue */
    color: white;
    padding: 2rem 1rem;
    text-align: center;
    box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

header h1 {
    font-size: 2.5rem;
    font-weight: 600;
    margin-bottom: 1rem;
}

nav a {
    text-decoration: none;
    color: white;
    background-color: #427fc5;
    padding: 10px;
    border-radius: 10px;
    transition: background-color 0.2s;
}

nav a:hover {
    background-color: #4075b2;
}

/* Main Content Area */
main {
    max-width: 1200px;
    margin: 2rem auto;
    padding: 0 1rem;
}

/* Lessons List Section (Card Style) */
#lessons-list {
    background-color: white;
    border-radius: 8px;
    padding: 2rem;
    box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}

#lessons-list h3 {
    font-size: 1.3rem;
    margin-bottom: 15px;
}

/* Responsive adjustments */
@media (max-width: 768px) {
    header h1 {
        font-size: 2rem;
    }
    
    #lessons-list {
        padding: 1.5rem;
    }
}

.pdf-container {
    margin-bottom: 1em;
}

.pdf-title {
    cursor: pointer;
    background-color: #f0f0f0;
    padding: 10px;
    margin: 15px;
    font-weight: bold;
    border: 1px solid #ccc;
    user-select: none;
    display: flex;
    justify-content: space-between;
    align-items: center;
    transition: background-color 0.2s;
}

.pdf-title:hover {
    background-color: #e0e0e0;
}

.pdf-title::after {
    content: '▲';
    font-size: 0.8rem;
    color: #666;
    transition: transform 0.3s ease;
}

.pdf-title.active::after {
    transform: rotate(180deg);
}

.pdf-frame {
    display: none;
    width: 100%;
    height: 700px;
    border: 1px solid #ccc;
    margin-top: 5px;
}