I'm having a little trouble with backgrounds here... I have an image set as my background, and I also selected 'black' as the default background color.
By doing this, I assumed that I was going to have the control page editor window show me my background image and allow me to place controls within its bounds, which it did. I also thought that if I viewed my control page on a web browser window larger than my background, that the excess would be black, which it isn't.
I tried deleting my background image, setting the background color to be black, and then making a static image at the bottom of the z-stack with what used to be my image. Everything still looks the same in my browser though - which is no good, since I have a black border around my background image, expecting that the rest of the web page would get filled in with black instead of white.