The tabs are YUI, there is an article about skinning YUI tabs here. There is currently only one skin available named sam, and we are hard coded to use it, but you could create one with the same class names and then add the css into the head of your layout.master below the other css, that way it will take presendence over settings above it from the YUI css we load from google CDN.

Not sure how tricky it will be do implement a YUI skin as I have never done it. I had the same problem with some of the included skins but I tweaked them to give darker font rather than figure out how to change the background in YUI.

Best,

Joe