Global Styles: Skip registration of variation styles when unsupported

lib/block-supports/block-style-variations.php

@@ -486,6 +486,13 @@ function gutenberg_register_block_style_variations_from_theme_json_data( $variat
  * @access private
  */
 function gutenberg_register_block_style_variations_from_theme() {
+	$has_partials_directory = is_dir( get_stylesheet_directory() . '/styles' ) || is_dir( get_template_directory() . '/styles' );
+
+	// Skip any registration of styles if no theme.json or variation partials.
+	if ( ! wp_theme_has_theme_json() && ! $has_partials_directory ) {
+		return;
+	}
+
 	// Partials from `/styles`.
 	$variations_partials = WP_Theme_JSON_Resolver_Gutenberg::get_style_variations( 'block' );
 	gutenberg_register_block_style_variations_from_theme_json_data( $variations_partials );