Well, as far as via XML, I don't know. I am familiar with XML, but I never bothered to look at the Poser UI stuff. However, I remember seeing that someone said you can change the main background color by clicking the paint bucket, then clicking on the background. This supposedly opens the color picker, which would allow you to change the background color. It's worth a shot, and much easier than trying to decipher the XML. Hope this helps.