Index: wp-content/themes/twentytwelve/js/theme-customizer.js
===================================================================
--- wp-content/themes/twentytwelve/js/theme-customizer.js	(revision 21615)
+++ wp-content/themes/twentytwelve/js/theme-customizer.js	(working copy)
@@ -33,14 +33,13 @@
 
 	// Hook into background color change and adjust body class value as needed.
 	wp.customize( 'background_color', function( value ) {
-		var body = $( 'body' );
 		value.bind( function( to ) {
 			if ( '#ffffff' == to || '#fff' == to || '' == to )
-				body.addClass( 'custom-background-white' );
+				$( 'body' ).addClass( 'custom-background-white' );
 			else if ( '' == to )
-				body.addClass( 'custom-background-empty' );
+				$( 'body' ).addClass( 'custom-background-empty' );
 			else
-				body.removeClass( 'custom-background-empty custom-background-white' );
+				$( 'body' ).removeClass( 'custom-background-empty custom-background-white' );
 		} );
 	} );
 } )( jQuery );
\ No newline at end of file