Index: wp-includes/class-wp-customize-setting.php
===================================================================
--- wp-includes/class-wp-customize-setting.php	(revision 21050)
+++ wp-includes/class-wp-customize-setting.php	(working copy)
@@ -382,7 +382,7 @@
  *
  * Results should be properly handled using another setting or callback.
  */
-class WP_Customize_Header_Image_Setting extends WP_Customize_Setting {
+final class WP_Customize_Header_Image_Setting extends WP_Customize_Setting {
 	public $id = 'header_image_data';
 
 	public function update( $value ) {
@@ -398,4 +398,12 @@
 		else
 			$custom_image_header->set_header_image( $value );
 	}
-}
\ No newline at end of file
+}
+
+final class WP_Customize_Background_Image_Setting extends WP_Customize_Setting {
+	public $id = 'background_image_thumb';
+
+	public function update( $value ) {
+		remove_theme_mod( 'background_image_thumb' );
+	}
+}
Index: wp-includes/class-wp-customize-manager.php
===================================================================
--- wp-includes/class-wp-customize-manager.php	(revision 21050)
+++ wp-includes/class-wp-customize-manager.php	(working copy)
@@ -781,6 +781,10 @@
 			'theme_supports' => 'custom-background',
 		) );
 
+		$this->add_setting( new WP_Customize_Background_Image_Setting( $this, 'background_image_thumb', array(
+			'theme_supports' => 'custom-background',
+		) ) );
+
 		$this->add_control( new WP_Customize_Background_Image_Control( $this ) );
 
 		$this->add_setting( 'background_repeat', array(