[Plugin: MarketPress – WordPress eCommerce] Sidebar widget "Category list"

Hi,
I have installed the MarketPress Lite plugin and when i activated the Categories list widget i saw that it gives me a long list of main Categoryes and their child with very little visual difference between the two.
As i was not able to add a dropdown for only child items of each category i am trying to introduce different background colour. One for Main Category and one for Child Category.
How can i do that?
Thank you.

http://wordpress.org/extend/plugins/wordpress-ecommerce/