Fixes from code review

src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php

@@ -297,7 +297,7 @@ public function update_item( $request ) {
 	 * @since 6.2.0 Added validation of styles.css property.
 	 *
 	 * @param WP_REST_Request $request Request object.
-	 * @return stdClass Changes to pass to wp_update_post.
+	 * @return stdClass|WP_Error Prepared item on success. WP_Error on when the custom CSS is not valid.
 	 */
 	protected function prepare_item_for_database( $request ) {
 		$changes     = new stdClass();
@@ -318,9 +318,11 @@ protected function prepare_item_for_database( $request ) {
 			$config = array();
 			if ( isset( $request['styles'] ) ) {
 				$config['styles'] = $request['styles'];
-				$validate_custom_css = $this->validate_custom_css( $request['styles']['css'] );
-				if ( is_wp_error( $validate_custom_css ) ) {
-					return $validate_custom_css;
+				if ( isset( $config['styles']['css'] ) ) {
+					$css_validation_result = $this->validate_custom_css( $config['styles']['css'] );
+					if ( is_wp_error( $css_validation_result ) ) {
+						return $css_validation_result;
+					}
 				}
 			} elseif ( isset( $existing_config['styles'] ) ) {
 				$config['styles'] = $existing_config['styles'];
@@ -681,7 +683,7 @@ private function validate_custom_css( $css ) {
 		if ( preg_match( '#</?\w+#', $css ) ) {
 			return new WP_Error(
 				'rest_custom_css_illegal_markup',
-				__( 'Markup is not allowed in CSS.', 'gutenberg' ),
+				__( 'Markup is not allowed in CSS.' ),
 				array( 'status' => 400 )
 			);
 		}

tests/phpunit/tests/theme/wpThemeJson.php

@@ -4540,7 +4540,7 @@ public function test_get_custom_css_handles_global_custom_css() {
 			)
 		);
 		$custom_css = 'body { color:purple; }';
-		$this->assertEquals( $custom_css, $theme_json->get_custom_css() );
+		$this->assertSame( $custom_css, $theme_json->get_custom_css() );
 	}
 
 	/**
@@ -4550,7 +4550,6 @@ public function test_get_custom_css_handles_global_custom_css() {
 	 *
 	 * @dataProvider data_custom_css_for_user_caps
 	 *
-	 *
 	 * @param string $user_property The property name for current user.
 	 * @param array  $expected      Expected results.
 	 */