Custom background fixes:

* Specify default background colors for the bundled themes.
* Change the default custom background callback to only operate on saved values, rather than default values.
* Prevent an unsaved default value from overriding a manually modified style.css file.

Props nacin, kobenland
fixes #20448


git-svn-id: https://develop.svn.wordpress.org/trunk@20973 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/theme.php

@@ -1102,8 +1102,9 @@ function background_color() {
  * @access protected
  */
 function _custom_background_cb() {
-	$background = get_background_image();
-	$color = get_background_color();
+	$background = get_theme_mod( 'background_image' );
+	$color = get_theme_mod( 'background_color' );
+
 	if ( ! $background && ! $color )
 		return;