/* オーロラのようにゆっくり色が揺らぐ背景 */
body {
    background-color: #0d0d17;
    background-image:
        radial-gradient(circle at 20% 20%, rgba(120, 80, 255, 0.35), transparent 40%),
        radial-gradient(circle at 80% 30%, rgba(0, 200, 255, 0.25), transparent 45%),
        radial-gradient(circle at 50% 80%, rgba(255, 80, 180, 0.2), transparent 50%);
    background-attachment: fixed;
    background-size: 200% 200%;
    animation: aurora-move 20s ease-in-out infinite;
    min-height: 100vh;
}

@keyframes aurora-move {
    0%   { background-position: 0% 0%, 100% 0%, 50% 100%; }
    50%  { background-position: 30% 20%, 70% 40%, 60% 70%; }
    100% { background-position: 0% 0%, 100% 0%, 50% 100%; }
}