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

body {
    font-family: Arial, sans-serif;
    min-height: 100vh;
    background-color: #2c3e50;
    color: #ecf0f1;
    display: flex;
    flex-direction: column;
}

.layout {
    display: flex;
    flex: 1;
}

.sidebar {
    width: 280px;
    background-color: #1a252f;
    display: flex;
    flex-direction: column;
    position: fixed;
    top: 0;
    left: 0;
    height: 100vh;
    z-index: 100;
}

.sidebar-header {
    padding: 20px;
    border-bottom: 1px solid #34495e;
}

.sidebar-header h2 {
    color: #3498db;
    font-size: 18px;
}

.sidebar-nav {
    flex: 1;
    overflow-y: auto;
}

.nav-item {
    border-bottom: 1px solid #34495e;
}

.nav-link {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 15px 20px;
    color: #ecf0f1;
    text-decoration: none;
    cursor: pointer;
    transition: background-color 0.3s;
}

.nav-link:hover {
    background-color: #34495e;
}

.nav-link.active {
    background-color: #3498db;
}

.nav-arrow {
    transition: transform 0.3s;
    font-size: 12px;
}

.nav-arrow.open {
    transform: rotate(180deg);
}

.chapter-nav {
    display: none;
    background-color: #243342;
}

.chapter-nav.open {
    display: block;
}

.chapter-title {
    padding: 12px 20px;
    color: #7f8c8d;
    font-size: 11px;
    text-transform: uppercase;
    letter-spacing: 1px;
    background-color: #1e2d3a;
}

.topic-link {
    display: flex;
    align-items: center;
    padding: 10px 20px 10px 25px;
    color: #bdc3c7;
    text-decoration: none;
    transition: all 0.3s;
    cursor: pointer;
    position: relative;
}

.topic-link:hover {
    background-color: #3498db;
    color: #ecf0f1;
}

.topic-link.active {
    background-color: #2980b9;
    color: #ecf0f1;
}

.topic-link.visited {
    color: #27ae60;
}

.topic-link.visited::before {
    content: '✓';
    position: absolute;
    right: 15px;
    color: #27ae60;
}

.topic-link.active.visited::before {
    color: #ecf0f1;
}

.topic-progress {
    font-size: 10px;
    color: #7f8c8d;
    padding: 5px 20px;
    background-color: #1e2d3a;
    border-top: 1px solid #34495e;
}

.sidebar-footer {
    border-top: 1px solid #34495e;
    background-color: #1a252f;
}

.feedback-link {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 15px 20px;
    color: #3498db;
    cursor: pointer;
    transition: background-color 0.3s;
}

.feedback-link:hover {
    background-color: #34495e;
}

.main-content {
    flex: 1;
    margin-left: 280px;
    padding: 20px;
    padding-bottom: 50px;
}

.section {
    display: none;
}

.section.active {
    display: block;
}