add_action( 'admin_enqueue_scripts', 'mytheme_admin_enqueue' );
function mytheme_admin_enqueue( $hook )
{
if( 'widgets.php' != $hook )
return;
wp_enqueue_script(
'my-script',
plugins_url( '/my-script.js', __FILE__ ),
array(), // dependencies
false, // version
true // on footer
);
wp_enqueue_style(
'my-style', plugins_url( '/my-style.css', __FILE__ )
);
}