Load the global styles before the theme styles in the editor

[oandregal]

Fixes https://core.trac.wordpress.org/ticket/55188
Related Gutenberg PR WordPress/gutenberg#37885

This follows what we do in the front-end, where theme styles are loaded after global styles by default.

How to test

• Use empty theme and apply the patch below via git apply <file.patch>:

Patch for empty theme

diff --git a/emptytheme/functions.php b/emptytheme/functions.php
index 5ee5593..a50baf4 100644
--- a/emptytheme/functions.php
+++ b/emptytheme/functions.php
@@ -6,8 +6,17 @@ if ( ! function_exists( 'emptytheme_support' ) ) :
 		// Adding support for core block visual styles.
 		add_theme_support( 'wp-block-styles' );
 
+		register_block_style(
+			'core/group',
+			array(
+				'name'  => 'background-color-hotpink',
+				'label' => 'BG Hotpink'
+			)
+		);
+
 		// Enqueue editor styles.
 		add_editor_style( 'style.css' );
+
 	}
 	add_action( 'after_setup_theme', 'emptytheme_support' );
 endif;
diff --git a/emptytheme/style.css b/emptytheme/style.css
index c10d92f..0dc53b7 100644
--- a/emptytheme/style.css
+++ b/emptytheme/style.css
@@ -14,3 +14,6 @@ Text Domain: emptytheme
 Emptytheme WordPress Theme, (C) 2021 WordPress.org
 Emptytheme is distributed under the terms of the GNU GPL.
 */
+.is-style-background-color-hotpink {
+    background-color: hotpink;
+}
\ No newline at end of file
diff --git a/emptytheme/theme.json b/emptytheme/theme.json
index 46b0979..1925b81 100644
--- a/emptytheme/theme.json
+++ b/emptytheme/theme.json
@@ -6,5 +6,14 @@
 			"contentSize": "840px",
 			"wideSize": "1100px"
 		}
+	},
+	"styles": {
+		"blocks": {
+			"core/group": {
+				"color": {
+					"background": "green"
+				}
+			}
+		}
 	}
 }

• Go to the editor and create a group block. Verify background color of the block is green.
• Go to the block sidebar and select the "BG Hotpink" style. Verify the background color of the block is now hotpink.

[oandregal]

I see this has been commited to trunk and backported to 5.9. https://core.trac.wordpress.org/ticket/55188 is now closed, so I'm closing this one as well.