@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&family=Noto+Sans+KR:wght@400;500;700&display=swap');

:root {
    --primary: #0f172a;      /* Deep Navy */
    --accent: #2563eb;       /* Vibrant Blue */
    --accent-light: #eff6ff;
    --text: #1e293b;         /* Slate 800 */
    --text-muted: #64748b;   /* Slate 500 */
    --bg: #ffffff;
    --bg-alt: #f8fafc;       /* Slate 50 */
    --border: #e2e8f0;       /* Slate 200 */
    --white: #ffffff;
    --radius-sm: 6px;
    --radius-md: 12px;
    --radius-lg: 20px;
    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
    --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);
}

* {
    box-sizing: border-box;
}

body {
    font-family: 'Inter', 'Noto Sans KR', sans-serif;
    margin: 0;
    background-color: var(--bg);
    color: var(--text);
    line-height: 1.5;
    -webkit-font-smoothing: antialiased;
}

/* Typography Overrides */
h1, h2, h3, h4 {
    margin: 0;
    font-weight: 700;
    color: var(--primary);
}

p {
    margin: 0;
    color: var(--text-muted);
}

/* Layout Utilities */
.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 1.5rem;
}

section {
    padding: 5rem 0;
}

/* Header */
header {
    background-color: rgba(255, 255, 255, 0.8);
    backdrop-filter: blur(8px);
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    z-index: 1000;
}

.header-container {
    height: 72px;
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.logo {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--primary);
    text-decoration: none;
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.logo span {
    color: var(--accent);
    font-weight: 400;
    font-size: 0.875rem;
}

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    padding: 0.75rem 1.5rem;
    font-weight: 600;
    border-radius: var(--radius-sm);
    transition: all 0.2s;
    cursor: pointer;
    text-decoration: none;
    border: none;
    font-size: 0.9375rem;
}

.btn-primary {
    background-color: var(--accent);
    color: var(--white);
    box-shadow: var(--shadow-sm);
}

.btn-primary:hover {
    background-color: #1d4ed8;
    transform: translateY(-1px);
    box-shadow: var(--shadow-md);
}

.btn-secondary {
    background-color: var(--white);
    color: var(--primary);
    border: 1px solid var(--border);
}

.btn-secondary:hover {
    background-color: var(--bg-alt);
    border-color: var(--text-muted);
}

/* Hero Section */
.hero {
    padding: 6rem 0;
    text-align: center;
    background: radial-gradient(circle at top right, var(--accent-light), transparent);
}

.hero h1 {
    font-size: 3.5rem;
    line-height: 1.1;
    margin-bottom: 1.5rem;
    letter-spacing: -0.025em;
}

.hero p {
    font-size: 1.25rem;
    max-width: 640px;
    margin: 0 auto 2.5rem;
    line-height: 1.6;
}

.hero-actions {
    display: flex;
    justify-content: center;
    gap: 1rem;
}

/* Feature/Benefit Section */
.features-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    gap: 2rem;
}

.feature-card {
    padding: 2.5rem;
    background: var(--bg-alt);
    border-radius: var(--radius-md);
    border: 1px solid var(--border);
    transition: transform 0.2s;
}

.feature-card:hover {
    transform: translateY(-4px);
}

.feature-icon {
    font-size: 2rem;
    margin-bottom: 1.25rem;
    display: block;
}

.feature-card h3 {
    margin-bottom: 0.75rem;
}

/* How It Works Section */
.how-it-works-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 4rem;
    position: relative;
}

.step-item {
    text-align: center;
}

.step-num {
    width: 48px;
    height: 48px;
    background: var(--accent);
    color: var(--white);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 700;
    margin: 0 auto 1.5rem;
}

/* Lesson Grid Premium */
.lessons-section {
    background-color: var(--bg-alt);
}

.section-title {
    text-align: center;
    margin-bottom: 3.5rem;
}

.section-title h2 {
    font-size: 2.25rem;
    margin-bottom: 1rem;
}

.lesson-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(350px, 1fr));
    gap: 2rem;
}

.lesson-card {
    background: var(--white);
    border-radius: var(--radius-md);
    overflow: hidden;
    border: 1px solid var(--border);
    transition: all 0.3s;
    display: flex;
    flex-direction: column;
}

.lesson-card:hover {
    box-shadow: var(--shadow-lg);
    transform: translateY(-4px);
}

.lesson-thumb {
    width: 100%;
    height: 200px;
    position: relative;
    background: #000;
}

.lesson-thumb img {
    width: 100%;
    height: 100%;
    object-fit: cover;
    opacity: 0.85;
}

.lesson-badge {
    position: absolute;
    top: 1rem;
    left: 1rem;
    padding: 0.25rem 0.75rem;
    background: rgba(15, 23, 42, 0.8);
    color: white;
    font-size: 0.75rem;
    font-weight: 600;
    border-radius: 100px;
    backdrop-filter: blur(4px);
}

.lesson-content {
    padding: 1.5rem;
    flex-grow: 1;
}

.lesson-meta {
    font-size: 0.8125rem;
    color: var(--accent);
    font-weight: 600;
    margin-bottom: 0.5rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.lesson-content h3 {
    margin-bottom: 0.75rem;
}

/* Learning Page Specific */
.learn-container {
    max-width: 1200px;
    margin: 2rem auto;
    display: grid;
    grid-template-columns: 1fr 400px;
    gap: 2rem;
}

.video-area {
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
}

.video-wrapper {
    position: relative;
    padding-bottom: 56.25%;
    height: 0;
    background: #000;
    border-radius: var(--radius-md);
    overflow: hidden;
    box-shadow: var(--shadow-lg);
}

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

.player-controls {
    display: flex;
    gap: 1rem;
    background: var(--bg-alt);
    padding: 1rem;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

/* Sidebar / Cards Area */
.sidebar-area {
    display: flex;
    flex-direction: column;
    gap: 2rem;
}

.cards-scroll-container {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.sentence-card {
    background: var(--white);
    padding: 1.25rem;
    border-radius: var(--radius-md);
    border: 1px solid var(--border);
    cursor: pointer;
    transition: all 0.2s;
    border-left: 4px solid transparent;
}

.sentence-card:hover {
    border-color: var(--accent);
    background-color: var(--accent-light);
}

.sentence-card.active {
    border-left-color: var(--accent);
    background-color: var(--accent-light);
}

.sentence-card h4 {
    font-size: 1.125rem;
    margin-bottom: 0.5rem;
}

.sentence-card p {
    font-size: 0.875rem;
}

/* Content Explanations Placeholder */
.explanation-section {
    margin-top: 3rem;
    padding-top: 3rem;
    border-top: 1px solid var(--border);
}

.explanation-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 2rem;
}

.exp-card {
    padding: 1.5rem;
    background: var(--bg-alt);
    border-radius: var(--radius-sm);
}

/* Footer */
footer {
    padding: 4rem 0;
    background: var(--primary);
    color: var(--white);
}

.footer-grid {
    display: grid;
    grid-template-columns: 2fr 1fr 1fr;
    gap: 4rem;
}

.footer-brand h4 { color: var(--white); margin-bottom: 1rem; }
.footer-brand p { color: #94a3b8; font-size: 0.875rem; }

/* Responsive */
@media (max-width: 1024px) {
    .learn-container {
        grid-template-columns: 1fr;
    }
    .hero h1 { font-size: 2.75rem; }
}

@media (max-width: 640px) {
    .hero h1 { font-size: 2.25rem; }
    .hero-actions { flex-direction: column; }
    .how-it-works-grid { gap: 2.5rem; }
    .footer-grid { grid-template-columns: 1fr; gap: 2rem; }
}