:root {
    --bg-dark: #121212;
    --bg-card: #1e1e1e;
    --text-main: #e0e0e0;
    --accent: #bb86fc;
    --secondary: #03dac6;
    --error: #cf6679;
}

body {
    background-color: var(--bg-dark);
    color: var(--text-main);
    font-family: 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
    line-height: 1.6;
    margin: 0;
    padding: 0;
}

.container {
    max-width: 900px;
    margin: 0 auto;
    padding: 40px 20px;
}

h1, h2, h3 {
    color: var(--accent);
}

.card {
    background: var(--bg-card);
    border-radius: 12px;
    padding: 24px;
    margin-bottom: 24px;
    box-shadow: 0 4px 20px rgba(0,0,0,0.3);
}

.btn {
    background: var(--accent);
    color: var(--bg-dark);
    border: none;
    padding: 10px 20px;
    border-radius: 6px;
    cursor: pointer;
    font-weight: bold;
    text-decoration: none;
    display: inline-block;
}

.btn:hover {
    filter: brightness(1.2);
}

.progress-container {
    background: #333;
    border-radius: 10px;
    height: 10px;
    margin: 10px 0;
}

.progress-bar {
    background: var(--secondary);
    height: 100%;
    border-radius: 10px;
    width: 0%;
    transition: width 0.5s ease;
}

.video-container {
    position: relative;
    padding-bottom: 56.25%;
    height: 0;
    overflow: hidden;
    margin-bottom: 20px;
}

.video-container iframe, .video-container video {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border-radius: 8px;
}

.quiz-option {
    display: block;
    background: #333;
    margin: 10px 0;
    padding: 12px;
    border-radius: 8px;
    cursor: pointer;
    transition: background 0.3s;
}

.quiz-option:hover {
    background: #444;
}

.quiz-option.correct {
    background: rgba(3, 218, 198, 0.2);
    border: 1px solid var(--secondary);
}

.nav-tabs { 
    display: flex; 
    gap: 15px; 
    margin: 25px 0;
    border-bottom: 1px solid #333;
    padding-bottom: 15px;
}

.tab-btn { 
    background: #252525; 
    color: #888; 
    border: 1px solid #444; 
    padding: 12px 24px; 
    cursor: pointer; 
    border-radius: 6px;
    font-weight: bold;
    transition: all 0.3s;
}

.tab-btn:hover {
    border-color: var(--accent);
    color: #fff;
}

.tab-btn.active { 
    background: var(--accent); 
    color: var(--bg-dark); 
    border-color: var(--accent);
    box-shadow: 0 0 15px rgba(187, 134, 252, 0.3);
}

.section { display: none; }
.section.active { display: block; animation: fadeIn 0.4s ease; }

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