Yup, you sure can.
I host the server my self and so I open up the GUI Server code and edit the HTML tags that are sent out along with the audio stream. Just search the code for #000000 which is the hex for the black background. I havn't tried this, but I have done exactly the same thing for the WWWinamp server as the generic black didn't quite look right on my while & pale blue scheme on my site!
Any questions post on my forum
http://www.blind-summit.co.uk/forum.html
Hope this helps / sends you in the right direction
Alex