gistfile1.css

body {
	background-image: url(%%bg1%%);
	background-repeat: no-repeat;
	background-color: black;
	background-position: 100% 10px;
	background-size: 40%;
}

body:before {
	content: "";
	opacity: 0;
	top: 0;
	left: 0;
	bottom: 0;
	right: 0;
	position: absolute;
	z-index: -1;
	
	background-image: url(%%bg2%%);
	background-repeat: no-repeat;
	background-color: black;
	background-position: 100% 10px;
	background-size: 40%;
	
	animation: body-bg 60s 15s infinite ease-out alternate;
}

@keyframes body-bg {
	0% {opacity: 0;}
	47% {opacity: 0;}
	53% {opacity: 1}
	100% {opacity: 1}
}