Index: wp-content/themes/twentyeleven/functions.php
===================================================================
--- wp-content/themes/twentyeleven/functions.php	(revision 20962)
+++ wp-content/themes/twentyeleven/functions.php	(working copy)
@@ -98,7 +98,7 @@
 	add_theme_support( 'post-formats', array( 'aside', 'link', 'gallery', 'status', 'quote', 'image' ) );
 
 	// Add support for custom backgrounds.
-	add_theme_support( 'custom-background' );
+	add_theme_support( 'custom-background', array( 'default-color' => 'FFF' ) );
 
 	// This theme uses Featured Images (also known as post thumbnails) for per-post/per-page Custom Header images
 	add_theme_support( 'post-thumbnails' );
Index: wp-content/themes/twentyten/functions.php
===================================================================
--- wp-content/themes/twentyten/functions.php	(revision 20962)
+++ wp-content/themes/twentyten/functions.php	(working copy)
@@ -94,8 +94,8 @@
 	) );
 
 	// This theme allows users to set a custom background.
-	add_theme_support( 'custom-background' );
-
+	add_theme_support( 'custom-background', array( 'default-color' => 'FFF' ) );
+	
 	// The custom header business starts here.
 
 	add_theme_support( 'custom-header', array(