[data-background-image] { position: relative; }

[data-background-image] .dark-overlay {

	/*	This is the element that is going to darken the background image
	By default, it's entirely black, but the JS code will give it
	some transparency by setting its opacity to a calculated value.
	The brighter the image, the higher the opacity, to compensate ! */

	content: "";
	display: block;
	position: absolute;
	top: 0; left: 0;
	width: 100%; height: 100%;
	background-color: black;

}