createBackground( slide, container ) {

		// Main slide background element
		let element = document.createElement( 'div' );
		element.className = 'slide-background ' + slide.className.replace( /present|past|future/, '' );

		// Inner background element that wraps images/videos/iframes
		let contentElement = document.createElement( 'div' );
		contentElement.className = 'slide-background-content';

		element.appendChild( contentElement );
		container.appendChild( element );

		slide.slideBackgroundElement = element;
		slide.slideBackgroundContentElement = contentElement;

		// Syncs the background to reflect all current background settings
		this.sync( slide );

		return element;

	}
Creates a background for the given slide.

@param {HTMLElement} slide
@param {HTMLElement} container The element that the background
should be appended to
@return {HTMLElement} New background div