Okay, it can be done but it's slightly tricky. You're going to have to know html very well and at least some asp. The OWA website files are stored in your exchange server at: C:\Program Files\Exchsrvr\exchweb\bin\usa. The file in that folder is the file that will need to be modified/customized to give it the look and feel you want. I just did some very minor changes to my site at
https://mail.redventures.net.