wp-calypso/client/reader/components/skeleton/style.scss

.skeleton {
	box-sizing: border-box;
	background-color: var( --studio-gray-5 );
	animation: skeleton-shimmer 1.5s infinite;
	border-radius: 4px;

	&--circle {
		border-radius: 50%;
	}

	@keyframes skeleton-shimmer {
		0% {
			opacity: 1;
		}
		50% {
			opacity: 0.5;
		}
		100% {
			opacity: 1;
		}
	}
}