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

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    color: #e2e8f0;
    background-color: #1a1f2c;
    line-height: 1.6;
    padding-bottom: 60px;
}

.skip-link {
    position: absolute;
    top: -40px;
    left: 0;
    background: #2b61ff;
    color: white;
    padding: 8px;
    z-index: 100;
    transition: top 0.2s;
    text-decoration: none;
    border-radius: 0 0 4px 0;
}

.skip-link:focus {
    top: 0;
}

.visually-hidden {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    border: 0;
}

.navbar {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background-color: #f1f5f9;
    padding: 12px 30px;
    border-bottom: 2px solid #cbd5e1;
    position: sticky;
    top: 0;
    z-index: 100;
}

.logo-container {
    display: flex;
    align-items: center;
}

.logo-img {
    height: 44px;
    width: auto;
    display: block;
}

.nav-links ul {
    display: flex;
    list-style: none;
}

.nav-links a {
    text-decoration: none;
    color: #475569;
    font-weight: 600;
    margin-left: 20px;
    font-size: 15px;
    transition: all 0.2s ease;
}

.lesson-container {
    max-width: 780px;
    margin: 40px auto;
    padding: 0 20px;
}

.overview-header {
    margin-bottom: 35px;
}

.badge {
    display: inline-block;
    background-color: rgba(43, 97, 255, 0.2);
    color: #60a5fa;
    font-size: 12px;
    font-weight: 700;
    padding: 4px 10px;
    border-radius: 20px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 12px;
}

.lesson-title {
    font-size: 36px;
    font-weight: 800;
    color: #ffffff;
    line-height: 1.2;
    margin-bottom: 15px;
}

.subtitle {
    font-size: 18px;
    color: #94a3b8;
}

.overview-section {
    margin-bottom: 40px;
}

.overview-section h2 {
    font-size: 24px;
    color: #ffffff;
    margin-bottom: 15px;
    font-weight: 700;
}

.overview-section p {
    font-size: 16px;
    color: #cbd5e1;
    margin-bottom: 20px;
}

.core-grid {
    display: flex;
    flex-direction: column;
    gap: 16px;
    margin-top: 20px;
}

.core-card {
    background-color: #242b3d;
    border: 1px solid #334155;
    padding: 24px;
    border-radius: 12px;
    transition: transform 0.2s ease;
}

.core-card h3 {
    color: #60a5fa;
    margin-bottom: 10px;
    font-size: 19px;
}

.core-card p {
    font-size: 14.5px;
    color: #94a3b8;
    margin-bottom: 0;
}

.media-section {
    margin: 35px 0;
}

.video-wrapper {
    position: relative;
    padding-bottom: 56.25%;
    height: 0;
    border-radius: 12px;
    overflow: hidden;
    box-shadow: 0 10px 30px rgba(0,0,0,0.4);
    background-color: #000000;
    border: 1px solid #334155;
}

.video-wrapper iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

.summary-box {
    background-color: rgba(43, 97, 255, 0.08);
    border-left: 4px solid #2b61ff;
    padding: 24px;
    border-radius: 0 12px 12px 0;
    margin: 40px 0;
}

.summary-box h3 {
    font-size: 16px;
    color: #60a5fa;
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 15px;
}

.summary-box ul {
    list-style-type: none;
}

.summary-box li {
    font-size: 15px;
    margin-bottom: 12px;
    padding-left: 20px;
    position: relative;
    color: #cbd5e1;
}

.summary-box li:last-child {
    margin-bottom: 0;
}

.summary-box li::before {
    content: "•";
    color: #2b61ff;
    font-weight: bold;
    position: absolute;
    left: 0;
}

.lesson-footer {
    margin-top: 45px;
    padding-top: 30px;
    border-top: 1px solid #334155;
}

.content-center {
    display: flex;
    justify-content: center;
}

.btn {
    display: inline-block;
    padding: 12px 24px;
    font-size: 15px;
    font-weight: 600;
    border-radius: 8px;
    cursor: pointer;
    border: none;
    text-decoration: none;
    text-align: center;
    transition: all 0.2s ease;
}

.btn-large {
    padding: 16px 36px;
    font-size: 16px;
    box-shadow: 0 4px 14px rgba(43, 97, 255, 0.3);
}

.btn-primary {
    background-color: #2b61ff;
    color: #ffffff;
}

@media (min-width: 640px) {
    .core-grid {
        flex-direction: row;
    }
    .core-card {
        flex: 1;
    }
}

.under-construction {
    background-color: #fef08a;
    border: 5px solid #eab308;
    color: #0f172a;
    padding: 10px 30px;
    font-size: 18px;
    text-align: center;
    font-weight: 700;
    width: 100%;
    letter-spacing: 0.3px;
    box-sizing: border-box;
}