SOURCE

body {
	background: linear-gradient(-45deg, #EE7752, #E73C7E, #23A6D5, #23D5AB);
	background-size: 400% 400%;
	animation: Gradient 15s ease infinite;
}

@keyframes Gradient {
	0% { background-position: 0% 50%; }
	50% { background-position: 100% 50%; }
	100% { background-position: 0% 50%; }
}