:root {
    --bg-deep: #05080f;
    --bg-slate: #0b1120;
    --bg-panel: rgba(255, 255, 255, 0.03);
    --border-glass: rgba(255, 255, 255, 0.08);
    --border-highlight: rgba(255, 255, 255, 0.15);
    
    --accent-blue: #22c55e;
    --accent-cyan: #10b981;
    --accent-indigo: #16a34a;
    --accent-success: #10b981;
    
    --text-main: #f8fafc;
    --text-muted: #94a3b8;
    --text-reading: #cbd5e1;
    
    --font-heading: 'Outfit', sans-serif;
    --font-body: 'Inter', sans-serif;
    
    --sidebar-width: 320px;
    --topbar-height: 70px;
}

body {
    background-color: var(--bg-deep);
    color: var(--text-main);
    font-family: var(--font-body);
    margin: 0;
    overflow: hidden; /* App-like layout */
}

/* App Layout Grid */
.lesson-layout {
    display: grid;
    grid-template-columns: var(--sidebar-width) 1fr;
    grid-template-rows: var(--topbar-height) 1fr;
    height: 100vh;
    width: 100vw;
}

/* Top Navigation Bar */
.lesson-topbar {
    grid-column: 1 / -1;
    grid-row: 1 / 2;
    background: var(--bg-deep);
    border-bottom: 1px solid var(--border-glass);
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0 2rem;
    z-index: 10;
}

.back-link {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    color: var(--text-muted);
    text-decoration: none;
    font-size: 0.9rem;
    transition: color 0.3s;
}

.back-link:hover {
    color: var(--text-main);
}

.course-progress {
    display: flex;
    align-items: center;
    gap: 1rem;
}

.progress-text {
    font-size: 0.85rem;
    color: var(--text-muted);
}

.progress-bar-container {
    width: 150px;
    height: 6px;
    background: rgba(255,255,255,0.1);
    border-radius: 100px;
    overflow: hidden;
}

.progress-bar-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--accent-indigo), var(--accent-cyan));
    width: 35%;
    border-radius: 100px;
    transition: width 0.5s ease-out;
}

/* Sidebar Syllabus */
.lesson-sidebar {
    grid-column: 1 / 2;
    grid-row: 2 / 3;
    background: var(--bg-slate);
    border-right: 1px solid var(--border-glass);
    overflow-y: auto;
    display: flex;
    flex-direction: column;
}

.sidebar-header {
    padding: 2rem 1.5rem;
    border-bottom: 1px solid var(--border-glass);
}

.sidebar-header h3 {
    font-size: 1.2rem;
    font-family: var(--font-heading);
    margin-bottom: 0.5rem;
    font-weight: 500;
}

.module-group {
    border-bottom: 1px solid var(--border-glass);
}

.module-title {
    padding: 1.5rem;
    font-size: 0.8rem;
    text-transform: uppercase;
    letter-spacing: 0.1em;
    color: var(--accent-cyan);
    font-weight: 600;
    display: flex;
    justify-content: space-between;
    align-items: center;
    cursor: pointer;
}

.lesson-list {
    list-style: none;
    padding: 0;
    margin: 0;
}

.lesson-item {
    padding: 1rem 1.5rem 1rem 3rem;
    display: flex;
    align-items: center;
    gap: 1rem;
    cursor: pointer;
    transition: all 0.2s;
    position: relative;
    color: var(--text-muted);
}

.lesson-item:hover {
    background: rgba(255,255,255,0.02);
    color: var(--text-main);
}

.lesson-item.active {
    background: rgba(16, 185, 129, 0.05);
    color: var(--text-main);
    border-left: 3px solid var(--accent-cyan);
}

.lesson-item.completed .status-icon {
    color: var(--accent-success);
}

.status-icon {
    font-size: 1.2rem;
    color: rgba(255,255,255,0.2);
}

.lesson-duration {
    margin-left: auto;
    font-size: 0.75rem;
    opacity: 0.5;
}

/* Main Content Area */
.lesson-main {
    grid-column: 2 / 3;
    grid-row: 2 / 3;
    background: var(--bg-deep);
    overflow-y: auto;
    position: relative;
    scroll-behavior: smooth;
}

.lesson-content-wrapper {
    max-width: 900px;
    margin: 0 auto;
    padding: 4rem 2rem 10rem;
}

.video-container {
    width: 100%;
    aspect-ratio: 16 / 9;
    background: #000;
    border-radius: 20px;
    border: 1px solid var(--border-glass);
    margin-bottom: 3rem;
    position: relative;
    overflow: hidden;
    display: flex;
    align-items: center;
    justify-content: center;
    box-shadow: 0 20px 50px rgba(0,0,0,0.5);
}

.video-placeholder {
    position: absolute;
    width: 100%;
    height: 100%;
    background: linear-gradient(145deg, #0f172a, #05080f);
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    color: var(--text-muted);
}

.play-btn {
    width: 80px;
    height: 80px;
    border-radius: 50%;
    background: rgba(16, 185, 129, 0.1);
    border: 1px solid var(--accent-cyan);
    color: var(--accent-cyan);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 2.5rem;
    cursor: pointer;
    transition: all 0.3s cubic-bezier(0.16, 1, 0.3, 1);
    backdrop-filter: blur(5px);
}

.play-btn:hover {
    transform: scale(1.1);
    background: rgba(16, 185, 129, 0.2);
    box-shadow: 0 0 30px rgba(16, 185, 129, 0.3);
}

.lesson-header {
    margin-bottom: 3rem;
}

.lesson-title {
    font-size: 2.5rem;
    font-family: var(--font-heading);
    margin-bottom: 1rem;
    color: #fff;
}

.lesson-description {
    font-size: 1.15rem;
    color: var(--text-reading);
    line-height: 1.7;
}

/* Reflection Integration */
.lesson-reflection {
    background: var(--bg-panel);
    border: 1px solid var(--border-highlight);
    border-radius: 24px;
    padding: 3rem;
    margin-top: 4rem;
    position: relative;
    overflow: hidden;
}

.lesson-reflection::before {
    content: '';
    position: absolute;
    top: 0; left: 0; width: 4px; height: 100%;
    background: linear-gradient(to bottom, var(--accent-cyan), var(--accent-indigo));
}

.reflection-prompt {
    font-size: 1.25rem;
    font-family: var(--font-heading);
    color: #fff;
    margin-bottom: 1.5rem;
}

.reflection-textarea {
    width: 100%;
    background: rgba(0,0,0,0.3);
    border: 1px solid var(--border-glass);
    border-radius: 12px;
    padding: 1.5rem;
    color: var(--text-main);
    font-family: var(--font-body);
    font-size: 1rem;
    line-height: 1.6;
    resize: vertical;
    min-height: 120px;
    outline: none;
    transition: border-color 0.3s;
}

.reflection-textarea:focus {
    border-color: var(--accent-cyan);
}

/* Action Footer */
.lesson-actions {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-top: 4rem;
    padding-top: 2rem;
    border-top: 1px solid var(--border-glass);
}

.btn-complete {
    background: #fff;
    color: var(--bg-deep);
    border: none;
    padding: 1rem 2.5rem;
    border-radius: 100px;
    font-weight: 600;
    font-size: 1rem;
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: 0.5rem;
    transition: all 0.3s;
}

.btn-complete:hover {
    transform: translateY(-2px);
    box-shadow: 0 10px 20px rgba(255,255,255,0.1);
}

.btn-secondary {
    background: transparent;
    color: var(--text-main);
    border: 1px solid var(--border-glass);
    padding: 1rem 2rem;
    border-radius: 100px;
    cursor: pointer;
    transition: all 0.3s;
}

.btn-secondary:hover {
    background: rgba(255,255,255,0.05);
}

/* Mobile Responsiveness */
@media (max-width: 900px) {
    .lesson-layout {
        grid-template-columns: 1fr;
    }
    .lesson-sidebar {
        display: none; /* In a real app, this would toggle open via a menu button */
    }
    .lesson-main {
        grid-column: 1 / -1;
    }
    .lesson-content-wrapper {
        padding: 2rem 1.5rem 6rem;
    }
    .lesson-actions {
        flex-direction: column;
        gap: 1rem;
    }
    .btn-complete, .btn-secondary {
        width: 100%;
        justify-content: center;
    }
}