Refactor to use :has() selector.

Instead of directly targeting the Featured Items blocks, we can use the
`:has()` selector to omit blocks that contain a child div with the
`has-background-dim` class in the editor.

Since browser support is not to where we need at the time of publishing
this, we can also add a temp fallback with `@supports`.

inc/customizer/class-storefront-customizer.php

@@ -1254,9 +1254,15 @@ public function block_editor_customizer_css() {
 				/* WP <=5.3 */
 				.editor-styles-wrapper .editor-block-list__block,
 				/* WP >=5.4 */
-				.editor-styles-wrapper .block-editor-block-list__block:not(.wp-block-woocommerce-featured-product):not(.wp-block-woocommerce-featured-category) {
+				.editor-styles-wrapper .block-editor-block-list__block:not(:has(div.has-background-dim)) {
 					color: ' . $storefront_theme_mods['text_color'] . ';
 				}
+				/* This following ruleset is a fallback for browsers that do not support the :has() selector. It can be removed once support reaches our requirements. */
+				@supports not (selector(:has(*))) {
+					.editor-styles-wrapper .block-editor-block-list__block:not(.wp-block-woocommerce-featured-product, .wp-block-woocommerce-featured-category) {
+						color: ' . $storefront_theme_mods['text_color'] . ';
+					}
+				}
 
 				.editor-styles-wrapper a,
 				.wp-block-freeform.block-library-rich-text__tinymce a {