Rivington: Clean up dim support in cover block.

rivington/style-editor.css

@@ -596,6 +596,12 @@ object {
 	/* Treating H2 separately to account for legacy /core styles */
 }
 
+.wp-block-cover.has-background-dim,
+.wp-block-cover-image.has-background-dim {
+	background-color: black;
+	color: white;
+}
+
 .wp-block-cover .wp-block-cover__inner-container,
 .wp-block-cover .wp-block-cover-image-text,
 .wp-block-cover .wp-block-cover-text,