Riley D. said,
Are you taking issue with the fact that there's empty space, or the fact that the empty space is grey? I'm open for suggestions on how to make it look more appealing :-)
I just saw this thread.
Of course I may be missing something, but actually, I wonder what is the intended purpose of the empty space beyond the Roll20's page canvas. I can't place anything there, nor even Cards. If that is the actual tabletop (different of the "canvas for background images and maps"), I think that it would be possible to place things on it, like any kind of virtual game assets.
Regardless of its purpose, my suggestion for this empty, grey space is an option for picking custom colors.