wp-calypso/client/layout/loader.scss

// If things take a bit to load...
.layout__loader {
	background: transparent;
	border-bottom: 1px solid transparent;
	height: var(--masterbar-height);
	margin-left: -10%;
	position: fixed;
	left: 50%;
	top: 0;
	width: 20%;
	z-index: z-index("root", ".layout__loader");

	// The transition-delay avoids showing the loader
	// too often on faster connections.
	display: none;
	opacity: 0;
	transition: opacity 0.1s linear;
	transition-delay: 0.8s;

	&.is-active {
		display: block;
		opacity: 1;
	}
}