SOURCE

<div class="rainbow"></div>

body {
	display: flex;
	justify-content: center;
	align-items: center;
	height: 100vh;
}

*, *::before, *::after {
	box-sizing: border-box;
}

@keyframes rotate {
	100% {
		transform: rotate(1turn);
	}
}

.rainbow {
	position: relative;
	z-index: 0;
	width: 400px;
	height: 300px;
	border-radius: 10px;
	padding: 2rem;
    overflow: hidden;
}
	
	.rainbow::before {
		content: '';
		position: absolute;
		z-index: -2;
		left: -50%;
		top: -50%;
		width: 200%;
		height: 200%;
		background-color: #399953;
		background-repeat: no-repeat;
		background-size: 50% 50%, 50% 50%;
		background-position: 0 0, 100% 0, 100% 100%, 0 100%;
		background-image: linear-gradient(#399953, #399953), linear-gradient(#fbb300, #fbb300), linear-gradient(#d53e33, #d53e33), linear-gradient(#377af5, #377af5);
        animation: rotate 4s linear infinite;
    }
	
	.rainbow::after {
		content: '';
		position: absolute;
		z-index: -1;
		left: 6px;
		top: 6px;
		width: calc(100% - 12px);
		height: calc(100% - 12px);
		border-radius: 5px;
        background: white;
        animation: opacityChange 3s infinite alternate;
	}


@keyframes opacityChange {
	50% {
		opacity:1;
	}
	100% {
		opacity: .5;
	}
}