Rosali, I think something simple like this would work. Since you also use it for your calendar plugins, just want to make sure this would work for you. This includes the whole jquery UI package, and im using the smoothness theme since I think it looks almost identical to the roundcube default theme.
class jqueryui extends rcube_plugin
{
public function init()
{
$this->add_hook('render_page', array($this, 'add_jqueryui'));
}
function add_jqueryui($args)
{
$skin_path = $this->local_skin_path();
// jquery UI
$this->include_script('js/jquery-ui-1.8.2.custom.min.js');
// jquery UI stylesheet
$this->include_stylesheet("$skin_path/css/smoothness/jquery-ui-1.8.2.custom.css");
}
}