bubbleSlideContrastClassToElement( slide, target ) {

		[ 'has-light-background', 'has-dark-background' ].forEach( classToBubble => {
			if( slide.classList.contains( classToBubble ) ) {
				target.classList.add( classToBubble );
			}
			else {
				target.classList.remove( classToBubble );
			}
		}, this );

	}
Bubble the 'has-light-background'/'has-dark-background' classes.