dropline bottom transparent until rollover

I am trying to make the dropline(bottom part of the menu) menu like this http://teatronaturale.it/. I am not sure how to make the dropline menu transparent/white until the parent is rolled over. I have made the dropline white to match my background. Is there any way to do this? This is an awesome template and i like that there is an override.css option!! Thank you in advance.

David

Sorry joomla 2.5, mysql, xampp, new version of gk_twn