:root {
    --bg-dark: #0f172a; /* Matched to main index */
    --card-bg: rgba(30, 41, 59, 0.7); /* Matched glass-card */
    --accent-c1: #22d3ee;
    --accent-c2: #fbbf24;
    --accent-all: #6366f1;
    --text-main: #f8fafc;
    --text-dim: #94a3b8;
    --glass-border: rgba(255, 255, 255, 0.1);
}

body {
    background-color: var(--bg-dark);
    color: var(--text-main);
    font-family: 'Inter', sans-serif;
    margin: 0;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

.hub-container {
    max-width: 1200px;
    margin: 0 auto;
    width: 100%;
    padding: 3rem 1.5rem;
    box-sizing: border-box;
    flex-grow: 1;
}

.header {
    text-align: center;
    margin-bottom: 3rem;
}

.header h1 {
    font-family: 'Outfit', sans-serif;
    font-size: 3rem;
    font-weight: 800;
    margin-bottom: 0.5rem;
    background: linear-gradient(to right, var(--accent-c1), var(--accent-all), var(--accent-c2));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

/* Randomize Buttons */
.randomize-actions {
    display: flex;
    justify-content: center;
    gap: 1.5rem;
    margin-bottom: 4rem;
    flex-wrap: wrap;
}

.random-btn {
    text-decoration: none;
    padding: 1rem 2rem;
    border-radius: 12px;
    font-weight: 700;
    font-size: 0.9rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    transition: all 0.3s;
    border: 1px solid rgba(255, 255, 255, 0.1);
    background: rgba(30, 41, 59, 0.5);
    backdrop-filter: blur(8px);
}

.random-btn.cyan { color: var(--accent-c1); border-color: rgba(34, 211, 238, 0.3); }
.random-btn.amber { color: var(--accent-c2); border-color: rgba(251, 191, 36, 0.3); }
.random-btn.indigo { color: var(--accent-all); border-color: rgba(99, 102, 241, 0.3); }

.random-btn.cyan:hover { background: var(--accent-c1); color: var(--bg-dark); box-shadow: 0 0 20px rgba(34, 211, 238, 0.4); }
.random-btn.amber:hover { background: var(--accent-c2); color: var(--bg-dark); box-shadow: 0 0 20px rgba(251, 191, 36, 0.4); }
.random-btn.indigo:hover { background: var(--accent-all); color: var(--bg-dark); box-shadow: 0 0 20px rgba(99, 102, 241, 0.4); }

/* Grid Layout */
.component-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(400px, 1fr));
    gap: 3rem;
}

.section-title {
    font-family: 'Outfit', sans-serif;
    display: flex;
    align-items: center;
    gap: 1rem;
    margin-bottom: 1.5rem;
    font-size: 1.5rem;
    font-weight: 700;
}

.c1-title { color: var(--accent-c1); }
.c2-title { color: var(--accent-c2); }

.unit-list {
    display: grid;
    grid-template-columns: 1fr;
    gap: 1rem;
}

.unit-card {
    background: var(--card-bg);
    backdrop-filter: blur(12px);
    border: 1px solid var(--glass-border);
    border-radius: 16px;
    margin-bottom: 1rem;
    overflow: hidden;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.unit-header {
    padding: 1.25rem 1.5rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
    cursor: pointer;
    transition: background 0.3s;
}

.unit-header:hover {
    background: rgba(255, 255, 255, 0.05);
}

.chevron {
    transition: transform 0.3s;
    opacity: 0.5;
}

.unit-card.active .chevron {
    transform: rotate(180deg);
    opacity: 1;
    color: var(--accent-all);
}

/* Sub-topic Drawer */
.topic-drawer {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.5s cubic-bezier(0.4, 0, 0.2, 1);
    background: rgba(15, 23, 42, 0.4);
    display: grid;
    grid-template-columns: 1fr;
}

.unit-card.active .topic-drawer {
    max-height: 800px; /* Large enough for all topics */
}

.topic-link {
    display: flex;
    align-items: center;
    gap: 1rem;
    padding: 1rem 1.5rem;
    text-decoration: none;
    color: var(--text-dim);
    font-size: 0.95rem;
    border-top: 1px solid rgba(255, 255, 255, 0.03);
    transition: all 0.2s;
}

.topic-link:hover {
    background: rgba(99, 102, 241, 0.1);
    color: white;
    padding-left: 2rem;
}

.topic-link i {
    width: 20px;
    text-align: center;
    font-size: 0.8rem;
    opacity: 0.7;
}

.topic-link.main-link {
    background: rgba(99, 102, 241, 0.15);
    color: white;
    font-weight: 700;
}

.unit-info {
    display: flex;
    flex-direction: column;
    gap: 0.25rem;
}

.unit-number {
    font-size: 0.75rem;
    font-weight: 800;
    opacity: 0.6;
    letter-spacing: 0.05em;
}

.unit-name {
    font-size: 1.1rem;
    font-weight: 600;
}

.mastery-value {
    font-size: 0.85rem;
    font-weight: 700;
    color: var(--text-dim);
    display: flex;
    align-items: center;
    gap: 0.75rem;
}

@media (max-width: 600px) {
    .component-grid { grid-template-columns: 1fr; }
    .randomize-actions { flex-direction: column; }
}