wp-calypso/client/dashboard/app/loading-screen.scss

@keyframes logo-pulse {
	0% {
		opacity: 0.65;
		filter: brightness(0.85);
	}
	50% {
		opacity: 1;
		filter: brightness(1.15);
	}
	100% {
		opacity: 0.65;
		filter: brightness(0.85);
	}
}

.wpcom-site__logo {
	position: absolute;
	top: 50%;
	left: 50%;
	transform: translate(-50%, -50%);
	margin: 0;
	fill: $gray-900;
	animation: logo-pulse 1.5s ease-in-out infinite;
}