:root {
    --primary-gradient: linear-gradient(135deg, #6366f1 0%, #a855f7 100%);
    --bg-color: #0f172a;
    --accent: #fcd34d;
    --success: #4ade80;
    --text-main: #f8fafc;
    --text-sub: #94a3b8;
}

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

body {
    font-family: 'Pretendard', sans-serif;
    background-color: var(--bg-color);
    background-image: 
        radial-gradient(at 0% 0%, hsla(253,16%,15%,1) 0, transparent 50%), 
        radial-gradient(at 100% 0%, hsla(339,49%,20%,1) 0, transparent 50%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    color: var(--text-main);
    padding: 1rem;
}

.glass-container {
    background: rgba(255, 255, 255, 0.05);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border: 1px solid rgba(255, 255, 255, 0.1);
    border-radius: 32px;
    padding: 2.5rem;
    width: 100%;
    max-width: 600px;
    box-shadow: 0 25px 50px -12px rgba(0, 0, 0, 0.5);
}

header h1 {
    font-size: clamp(2rem, 8vw, 2.5rem);
    text-align: center;
    margin-bottom: 0.5rem;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    font-weight: 800;
}

.progress-info {
    text-align: center;
    color: var(--text-sub);
    margin-bottom: 1.5rem;
    font-weight: 500;
}

.level-selector {
    display: flex;
    gap: 8px;
    margin-bottom: 1.5rem;
}

.level-selector button {
    flex: 1;
    padding: 0.7rem;
    border-radius: 12px;
    border: 1px solid rgba(255, 255, 255, 0.1);
    background: rgba(255, 255, 255, 0.05);
    color: var(--text-main);
    cursor: pointer;
    transition: 0.3s;
    font-weight: 600;
}

.level-selector button.active {
    background: var(--primary-gradient);
    border-color: transparent;
}

.mode-switch {
    display: flex;
    gap: 8px;
    margin-bottom: 1.5rem;
}

.mode-btn {
    flex: 1;
    padding: 0.6rem;
    font-size: 0.85rem;
    border-radius: 10px;
    border: 1px solid rgba(255, 255, 255, 0.1);
    background: rgba(255, 255, 255, 0.03);
    color: var(--text-sub);
    cursor: pointer;
    transition: 0.3s;
}

.mode-btn.active {
    background: var(--success);
    color: #0f172a;
    font-weight: 700;
    border-color: transparent;
}

.study-card {
    background: rgba(0, 0, 0, 0.25);
    border-radius: 24px;
    padding: 1.5rem;
    margin-bottom: 1.5rem;
}

.card-header {
    display: flex;
    justify-content: space-between;
    margin-bottom: 1.5rem;
    color: var(--accent);
    font-weight: 700;
}

.sentence-item {
    background: rgba(255, 255, 255, 0.03);
    border-radius: 16px;
    padding: 1.2rem;
    margin-bottom: 1rem;
    transition: 0.3s;
    border: 1px solid transparent;
}

.sentence-item:hover {
    background: rgba(255, 255, 255, 0.07);
}

.sentence-item.mastered-ui {
    border-color: var(--success);
    background: rgba(74, 222, 128, 0.05);
}

.content {
    cursor: pointer;
}

.en {
    display: block;
    font-size: 1.2rem;
    font-weight: 700;
    margin-bottom: 0.5rem;
}

.ko {
    display: none;
    color: var(--text-sub);
    font-size: 1rem;
}

.sentence-item.revealed .ko {
    display: block;
    animation: fadeIn 0.3s;
}

.sentence-actions {
    display: flex;
    gap: 10px;
    margin-top: 1rem;
    border-top: 1px solid rgba(255, 255, 255, 0.05);
    padding-top: 0.8rem;
}

.icon-btn {
    background: rgba(255, 255, 255, 0.08);
    border: none;
    color: white;
    padding: 6px 14px;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.85rem;
    transition: 0.2s;
    font-weight: 600;
}

.icon-btn:hover {
    background: rgba(255, 255, 255, 0.15);
}

.check-btn.mastered {
    background: var(--success);
    color: #0f172a;
}

.progress-bar-container {
    height: 6px;
    background: rgba(255, 255, 255, 0.1);
    border-radius: 3px;
    margin-top: 1rem;
}

.progress-fill {
    height: 100%;
    background: var(--success);
    width: 0%;
    transition: width 0.5s ease;
}

.controls {
    display: flex;
    gap: 12px;
}

.primary-btn {
    flex: 2;
    background: var(--primary-gradient);
    color: white;
    border: none;
    padding: 1.2rem;
    font-size: 1.1rem;
    font-weight: 700;
    border-radius: 16px;
    cursor: pointer;
    transition: 0.3s;
}

.secondary-btn {
    flex: 1;
    background: rgba(255, 255, 255, 0.1);
    color: var(--text-main);
    border: none;
    padding: 1.2rem;
    border-radius: 16px;
    cursor: pointer;
}

footer {
    margin-top: 1.5rem;
    text-align: center;
    color: var(--text-sub);
    font-size: 0.85rem;
    line-height: 1.5;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(5px); }
    to { opacity: 1; transform: translateY(0); }
}

@media (max-width: 480px) {
    .glass-container { padding: 1.5rem; }
    .controls { flex-direction: column; }
}