Theme JSON: replace top-level background style objects on merge (#66656)

This commit fixes an omission in the theme json merge logic where top-level background image objects are not replaced, rather they are merged, which was the state of affairs before #64128

Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

backport-changelog/6.8/7697.md

@@ -0,0 +1,3 @@
+https://github.com/WordPress/wordpress-develop/pull/7697
+
+* https://github.com/WordPress/gutenberg/pull/66656

lib/class-wp-theme-json-gutenberg.php

@@ -3297,6 +3297,10 @@ public function merge( $incoming ) {
 			array(),
 			array( 'include_node_paths_only' => true )
 		);
+
+		// Add top-level styles.
+		$style_nodes[] = array( 'path' => array( 'styles' ) );
+
 		foreach ( $style_nodes as $style_node ) {
 			$path = $style_node['path'];
 			/*

phpunit/class-wp-theme-json-test.php

@@ -2301,7 +2301,9 @@ public function test_merge_incoming_background_styles() {
 				'styles'  => array(
 					'background' => array(
 						'backgroundImage' => array(
-							'url' => 'http://example.org/quote.png',
+							'id'     => 'uploaded',
+							'source' => 'file',
+							'url'    => 'http://example.org/quote.png',
 						),
 						'backgroundSize'  => 'cover',
 					),
@@ -2333,7 +2335,10 @@ public function test_merge_incoming_background_styles() {
 			'version' => WP_Theme_JSON_Gutenberg::LATEST_SCHEMA,
 			'styles'  => array(
 				'background' => array(
-					'backgroundSize' => 'contain',
+					'backgroundImage' => array(
+						'url' => 'http://example.org/site.png',
+					),
+					'backgroundSize'  => 'contain',
 				),
 				'blocks'     => array(
 					'core/group' => array(
@@ -2363,7 +2368,7 @@ public function test_merge_incoming_background_styles() {
 			'styles'  => array(
 				'background' => array(
 					'backgroundImage' => array(
-						'url' => 'http://example.org/quote.png',
+						'url' => 'http://example.org/site.png',
 					),
 					'backgroundSize'  => 'contain',
 				),