Unfortunately, you can't change the CSS to Eventbrite's iFrame widgets.
You can replicated the features of the Widget and customize to your liking with Eventbrite's API.
Here is the developer center: http://developer.eventbrite.com/
You'll want to use this API method, /user_list_events: http://developer.eventbrite.com/doc/users/user_list_events/
Hope that helps!