Ajax callback for 'css'.
function ajax_forms_test_advanced_commands_css_callback($form, $form_state) {
$selector = '#css_div';
$color = 'blue';
$commands = array();
$commands[] = ajax_command_css($selector, array(
'background-color' => $color,
));
return array(
'#type' => 'ajax',
'#commands' => $commands,
);
}