Trouble styling calendar plugin--ATA over-riding CSS?

I have Kieran O'Shea's calendar plugin working on my site, and in fact have it working on the same page as our old Perl script calendar. Look at the bottom of this page for it: http://www.larchmontgazette.com/?page_id=131

You see the ugly gray or blue blocks for the events? For the life of me I can't get rid of those. I do want colors to show different categories of events, as in the old calendar. What I want is for the colors to be backgrounds for the events as in the old calendar, not those thick blocks. Even using Firebug, I can't figure out what to change. When I try to change the plugin's CSS, nothing happens. I think something in ATA must be over-riding it, but I can't figure it out. Help!