Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 39531)
+++ src/wp-includes/theme.php	(working copy)
@@ -1554,7 +1554,7 @@
 	$style = $color ? "background-color: #$color;" : '';
 
 	if ( $background ) {
-		$image = " background-image: url(" . wp_json_encode( $background ) . ");";
+		$image = " background-image: url(" . esc_url( $background ) . ");";
 
 		// Background Position.
 		$position_x = get_theme_mod( 'background_position_x', get_theme_support( 'custom-background', 'default-position-x' ) );