body {
    opacity: 0;
}

body.fade-in {
    animation: fadeIn 0.3s ease-in forwards;
}

body.fade-out {
    animation: fadeOut 0.3s ease-out forwards;
}

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

@keyframes fadeOut {
    from { opacity: 1; }
    to { opacity: 0; }
}