add --slide-width/height css variables

js/reveal.js

@@ -421,6 +421,10 @@ export default function( revealElement, options ) {
 		dom.wrapper.setAttribute( 'data-transition-speed', config.transitionSpeed );
 		dom.wrapper.setAttribute( 'data-background-transition', config.backgroundTransition );
 
+		// Expose our configured slide dimensions as custom props
+		dom.viewport.style.setProperty( '--size-width', config.width + 'px' );
+		dom.viewport.style.setProperty( '--size-height', config.height + 'px' );
+
 		if( config.shuffle ) {
 			shuffle();
 		}