:root {
    --primary-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --accent-color: #ffd700;
    --glass-bg: rgba(255, 255, 255, 0.1);
    --glass-border: rgba(255, 255, 255, 0.2);
    --shadow-deep: 0 10px 30px rgba(0, 0, 0, 0.3);
    --bg-main: #0f172a;
    --bg-radial-1: #1e293b;
    --bg-radial-2: #0f172a;
    --text-color: white;
    --sub-text: #94a3b8;
    --heading-gradient: linear-gradient(to right, #fff, #cbd5e1);
    --ball-bg: white;
    --ball-text: #1e293b;
}

[data-theme="light"] {
    --glass-bg: rgba(255, 255, 255, 0.6);
    --glass-border: rgba(0, 0, 0, 0.1);
    --shadow-deep: 0 10px 30px rgba(0, 0, 0, 0.1);
    --bg-main: #e2e8f0;
    --bg-radial-1: #f1f5f9;
    --bg-radial-2: #e2e8f0;
    --text-color: #1e293b;
    --sub-text: #64748b;
    --heading-gradient: linear-gradient(to right, #1e293b, #475569);
    --ball-bg: #ffffff;
    --ball-text: #1e293b;
}

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

body {
    font-family: 'Pretendard', -apple-system, BlinkMacSystemFont, system-ui, Roboto, sans-serif;
    background: var(--bg-main);
    color: var(--text-color);
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    overflow: hidden;
    transition: background 0.4s, color 0.4s;
}

.bg-gradient {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: radial-gradient(circle at top right, var(--bg-radial-1), var(--bg-radial-2));
    transition: background 0.4s;
    z-index: -1;
}

.main-container {
    background: var(--glass-bg);
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
    border: 1px solid var(--glass-border);
    padding: 3rem;
    position: relative;
    transition: background 0.4s, border-color 0.4s, box-shadow 0.4s;
    border-radius: 2rem;
    text-align: center;
    box-shadow: var(--shadow-deep);
    width: 90%;
    max-width: 600px;
    animation: fadeIn 0.8s ease-out;
}

header h1 {
    font-size: 3rem;
    background: var(--heading-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    margin-bottom: 0.5rem;
}

header p {
    color: var(--sub-text);
    margin-bottom: 2.5rem;
}

.number-container {
    display: flex;
    justify-content: center;
    gap: 1rem;
    margin-bottom: 3rem;
    flex-wrap: wrap;
}

.ball {
    width: 60px;
    height: 60px;
    background: var(--ball-bg);
    color: var(--ball-text);
    border-radius: 50%;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 1.5rem;
    font-weight: 800;
    box-shadow: 0 4px 15px rgba(0,0,0,0.2);
    transition: transform 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275), background 0.3s;
}

.ball.active {
    transform: scale(1.1);
    background: var(--accent-color);
}

.glow-button {
    background: var(--primary-gradient);
    border: none;
    color: white;
    padding: 1rem 3rem;
    font-size: 1.2rem;
    font-weight: 700;
    border-radius: 1rem;
    cursor: pointer;
    transition: all 0.3s;
    box-shadow: 0 0 20px rgba(102, 126, 234, 0.4);
}

.glow-button:hover {
    transform: translateY(-2px);
    box-shadow: 0 0 30px rgba(102, 126, 234, 0.6);
}

.glow-button:active {
    transform: translateY(1px);
}

.theme-toggle {
    position: absolute;
    top: 1.2rem;
    right: 1.2rem;
    width: 48px;
    height: 48px;
    border-radius: 50%;
    border: 1px solid var(--glass-border);
    background: var(--glass-bg);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.3rem;
    transition: all 0.3s;
    box-shadow: 0 2px 10px rgba(0, 0, 0, 0.15);
}

.theme-toggle:hover {
    transform: scale(1.1);
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.25);
}

.icon-sun,
.icon-moon {
    position: absolute;
    transition: opacity 0.3s, transform 0.3s;
}

/* Dark mode (default): show sun icon */
.icon-sun { opacity: 1; transform: rotate(0deg); }
.icon-moon { opacity: 0; transform: rotate(-90deg); }

/* Light mode: show moon icon */
[data-theme="light"] .icon-sun { opacity: 0; transform: rotate(90deg); }
[data-theme="light"] .icon-moon { opacity: 1; transform: rotate(0deg); }

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(20px); }
    to { opacity: 1; transform: translateY(0); }
}

@media (max-width: 480px) {
    .ball {
        width: 45px;
        height: 45px;
        font-size: 1.2rem;
    }
}