/* ===== CSS Variables ===== */
:root {
    --bg-primary: #0f0f1a;
    --bg-secondary: #1a1a2e;
    --bg-card: #16213e;
    --accent: #00d9ff;
    --accent-hover: #00b8d9;
    --text-primary: #ffffff;
    --text-secondary: #a0a0b0;
    --text-muted: #6b6b80;
    --border: #2a2a4a;
    --success: #4ade80;
    --warning: #fbbf24;
    --error: #f87171;
    --shadow: 0 4px 20px rgba(0, 0, 0, 0.3);
    --radius: 12px;
    --radius-sm: 8px;
    --transition: 0.3s ease;
}

/* ===== Reset & Base ===== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
}

/* ===== Header ===== */
.header {
    background: linear-gradient(135deg, var(--bg-secondary) 0%, var(--bg-primary) 100%);
    padding: 40px 0;
    text-align: center;
    border-bottom: 1px solid var(--border);
}

.logo {
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--accent);
    letter-spacing: 2px;
    margin-bottom: 8px;
}

.subtitle {
    color: var(--text-secondary);
    font-size: 1.1rem;
}

/* ===== Back Navigation ===== */
.back-nav {
    padding: 20px 0;
}

.back-link {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    color: var(--accent);
    text-decoration: none;
    font-weight: 500;
    transition: var(--transition);
}

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

/* ===== Main Content ===== */
main {
    padding: 40px 0;
}

section {
    margin-bottom: 50px;
}

h2 {
    font-size: 1.8rem;
    margin-bottom: 20px;
    margin-top: 40px;
    color: var(--text-primary);
    border-left: 4px solid var(--accent);
    padding-left: 16px;
}

/* ===== Intro Section ===== */
.intro p {
    color: var(--text-secondary);
    font-size: 1.1rem;
    max-width: 700px;
}

/* ===== Topic Grid ===== */
.topic-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 24px;
}

.topic-card {
    background: var(--bg-card);
    border-radius: var(--radius);
    padding: 24px;
    border: 1px solid var(--border);
    transition: var(--transition);
    position: relative;
    overflow: hidden;
}

.topic-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 3px;
    background: linear-gradient(90deg, var(--accent), transparent);
    opacity: 0;
    transition: var(--transition);
}

.topic-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow);
    border-color: var(--accent);
}

.topic-card:hover::before {
    opacity: 1;
}

.topic-card.hidden {
    display: none;
}

.topic-icon {
    font-size: 2.5rem;
    margin-bottom: 16px;
}

.topic-card h3 {
    font-size: 1.3rem;
    margin-bottom: 8px;
    color: var(--text-primary);
}

.topic-card p {
    color: var(--text-secondary);
    margin-bottom: 16px;
    font-size: 0.95rem;
}

.topic-link {
    display: inline-block;
    color: var(--accent);
    text-decoration: none;
    font-weight: 500;
    transition: var(--transition);
}

.topic-link:hover {
    color: var(--accent-hover);
}

/* ===== Section Cards ===== */
.section-card {
    display: block;
    background: var(--bg-card);
    border-radius: var(--radius);
    padding: 28px;
    border: 1px solid var(--border);
    transition: var(--transition);
    text-decoration: none;
    position: relative;
    overflow: hidden;
}

.section-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 3px;
    background: linear-gradient(90deg, var(--accent), transparent);
    opacity: 0;
    transition: var(--transition);
}

.section-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow);
    border-color: var(--accent);
}

.section-card:hover::before {
    opacity: 1;
}

.section-card h3 {
    font-size: 1.4rem;
    margin-bottom: 10px;
    color: var(--text-primary);
}

.section-card p {
    color: var(--text-secondary);
    margin-bottom: 16px;
    font-size: 0.95rem;
    line-height: 1.5;
}

.section-card .task-count {
    display: inline-block;
    background: var(--bg-secondary);
    color: var(--accent);
    padding: 4px 12px;
    border-radius: 20px;
    font-size: 0.85rem;
    font-weight: 500;
}

/* ===== Task List in Section ===== */
.task-list {
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.task-item {
    display: block;
    background: var(--bg-card);
    border-radius: var(--radius);
    padding: 20px 24px;
    border: 1px solid var(--border);
    transition: var(--transition);
    text-decoration: none;
}

.task-item:hover {
    border-color: var(--accent);
    transform: translateX(8px);
}

.task-item h3 {
    font-size: 1.2rem;
    color: var(--text-primary);
    margin-bottom: 6px;
}

.task-item p {
    color: var(--text-secondary);
    font-size: 0.9rem;
}

/* ===== Category Buttons ===== */
.category-buttons {
    display: flex;
    flex-wrap: wrap;
    gap: 12px;
}

.category-btn {
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    color: var(--text-secondary);
    padding: 10px 20px;
    border-radius: var(--radius-sm);
    cursor: pointer;
    font-size: 0.95rem;
    font-weight: 500;
    transition: var(--transition);
}

.category-btn:hover {
    border-color: var(--accent);
    color: var(--text-primary);
}

.category-btn.active {
    background: var(--accent);
    border-color: var(--accent);
    color: var(--bg-primary);
}

/* ===== Task Detail Page ===== */
.task-header {
    background: linear-gradient(135deg, var(--bg-secondary) 0%, var(--bg-primary) 100%);
    padding: 40px 0;
    border-bottom: 1px solid var(--border);
}

.task-title {
    font-size: 2rem;
    color: var(--text-primary);
    margin-bottom: 8px;
}

.task-subtitle {
    color: var(--accent);
    font-size: 1.1rem;
}

.task-content {
    padding: 40px 0;
}

.task-section {
    background: var(--bg-card);
    border-radius: var(--radius);
    padding: 24px;
    margin-bottom: 24px;
    border: 1px solid var(--border);
}

.task-section h3 {
    font-size: 1.3rem;
    margin-bottom: 16px;
    color: var(--accent);
    display: flex;
    align-items: center;
    gap: 10px;
}

.task-section p {
    color: var(--text-secondary);
    line-height: 1.8;
}

.task-section ul {
    color: var(--text-secondary);
    padding-left: 24px;
    line-height: 2;
}

.task-section li {
    margin-bottom: 8px;
}

/* ===== Code Blocks ===== */
.code-block {
    background: var(--bg-primary);
    border-radius: var(--radius-sm);
    padding: 20px;
    margin-top: 16px;
    overflow-x: auto;
    border: 1px solid var(--border);
}

.code-block pre {
    font-family: 'JetBrains Mono', 'Fira Code', monospace;
    font-size: 0.9rem;
    color: var(--text-primary);
    white-space: pre-wrap;
    word-break: break-word;
}

.code-label {
    display: inline-block;
    background: var(--accent);
    color: var(--bg-primary);
    padding: 4px 12px;
    border-radius: 4px;
    font-size: 0.8rem;
    font-weight: 600;
    margin-bottom: 12px;
    text-transform: uppercase;
}

.code-input {
    color: var(--success);
}

.code-output {
    color: var(--warning);
}

.code-comment {
    color: var(--text-muted);
    font-style: italic;
}

/* ===== Info Box ===== */
.info-box {
    background: rgba(0, 217, 255, 0.1);
    border-left: 4px solid var(--accent);
    padding: 16px 20px;
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    margin: 16px 0;
}

.info-box p {
    color: var(--text-primary) !important;
}

/* ===== Footer ===== */
.footer {
    background: var(--bg-secondary);
    padding: 30px 0;
    text-align: center;
    border-top: 1px solid var(--border);
    margin-top: 40px;
}

.footer p {
    color: var(--text-muted);
    font-size: 0.9rem;
}

/* ===== Navigation Between Tasks ===== */
.task-nav {
    display: flex;
    justify-content: space-between;
    gap: 20px;
    margin-top: 40px;
    padding-top: 30px;
    border-top: 1px solid var(--border);
}

.nav-btn {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 12px 20px;
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
    color: var(--text-secondary);
    text-decoration: none;
    transition: var(--transition);
    font-size: 0.95rem;
}

.nav-btn:hover {
    border-color: var(--accent);
    color: var(--accent);
}

.nav-btn.disabled {
    opacity: 0.5;
    pointer-events: none;
}

/* ===== Responsive ===== */
@media (max-width: 768px) {
    .header {
        padding: 30px 0;
    }

    .logo {
        font-size: 2rem;
    }

    .subtitle {
        font-size: 1rem;
    }

    h2 {
        font-size: 1.5rem;
    }

    .topic-grid {
        grid-template-columns: 1fr;
        gap: 16px;
    }

    .topic-card {
        padding: 20px;
    }

    .category-buttons {
        gap: 8px;
    }

    .category-btn {
        padding: 8px 16px;
        font-size: 0.85rem;
    }

    .task-title {
        font-size: 1.5rem;
    }

    .task-section {
        padding: 20px;
    }

    .code-block {
        padding: 16px;
    }

    .code-block pre {
        font-size: 0.8rem;
    }

    .task-nav {
        flex-direction: column;
    }

    .nav-btn {
        justify-content: center;
    }
}

@media (max-width: 480px) {
    .container {
        padding: 0 16px;
    }

    .logo {
        font-size: 1.8rem;
    }

    h2 {
        font-size: 1.3rem;
        padding-left: 12px;
    }

    .topic-icon {
        font-size: 2rem;
    }

    .topic-card h3 {
        font-size: 1.1rem;
    }
}