Ajax form callback: Selects 'css'.
function ajax_forms_test_advanced_commands_css_callback($form, $form_state) {
$selector = '#css_div';
$color = 'blue';
$response = new AjaxResponse();
$response
->addCommand(new Ajax\CssCommand($selector, array(
'background-color' => $color,
)));
return $response;
}