wp-calypso/client/dashboard/components/loading-line/style.scss

@keyframes loading {
	from {
		transform: translateX(-50vw);
	}
	to {
		transform: translateX(100vw);
	}
}

.loading-line-wrapper {
	position: fixed;
	top: 0;
	left: 0;
	width: 100%;
	height: 2px;
	background-color: transparent;
	z-index: 1000;
}

.loading-line-bar {
	width: 50vw;
	height: 100%;
	position: absolute;
	background: linear-gradient( to right, transparent 0%, var(--wp-admin-theme-color) 50%, transparent 100% );
	animation: loading 2s infinite linear;
}