html,
body,
.box {
	height: 100%;
	display: flex;
	align-items: center;
	justify-content: center;
}

body {
	background-color: #222;
}

.box {
	font-size: 2.5em;
	width: 10em;
	height: 5em;
	background-color: #111;
	border-radius: 0.5em;
	text-align: center;
	line-height: 1.5em;
	font-family: sans-serif;
	position: relative;
	animation: animate_text 2s linear infinite alternate;
}

.box::before {
	content: '';
	position: absolute;
	width: calc(100% + 10%);
	height: calc(100% + 20%);
	border-radius: 0.5em;
	z-index: -1;
	background-image: linear-gradient(60deg, aquamarine, cornflowerblue, goldenrod, hotpink, salmon, lightgreen, sandybrown, violet);
	background-size: 300%;
	animation: animate_bg 5s infinite;
}

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

	50% {
		background-position: 100%, 50%;
	}
}

@keyframes animate_text {
	from { 
		color: lime;
	}

	to {
		color: yellow;
	}
}