Use HTML Tag Processor for STYLE tags in theme.json

Author: sirreal

src/wp-includes/theme.php

@@ -1953,11 +1953,13 @@ function _custom_background_cb() {
 
 		$style .= $image . $position . $size . $repeat . $attachment;
 	}
-	?>
-<style<?php echo $type_attr; ?> id="custom-background-css">
-body.custom-background { <?php echo trim( $style ); ?> }
-</style>
-	<?php
+
+	$processor = new WP_HTML_Tag_Processor( "<style{$type_attr} id=\"custom-background-css\"></style>" );
+	$processor->next_tag();
+
+	$style_tag_content = 'body.custom-background { ' . trim( $style ) . ' }';
+	$processor->set_modifiable_text( "\n{$style_tag_content}\n" );
+	echo $processor->get_updated_html();
 }
 
 /**