Ticket #28687: 28687.5.diff

src/wp-includes/post-template.php

@@ -646 +646 @@
                 $classes[] = 'no-customize-support';
         }
 
-        if ( get_theme_mod( 'background_color' ) || get_background_image() )
+        if ( get_background_color() !== get_theme_support( 'custom-background', 'default-color' ) || get_background_image() )
                 $classes[] = 'custom-background';
 
         $page = $wp_query->get( 'page' );