In theory, you can pass to the sketch the parameter --bgcolor=#xxxxxx
But my little attempts right now weren't successful, looks like the parameters are ignored.
My best attempt was to put the parameters after the class name in the lib/args.txt file: at least I can see them in my sketch passing the arguments to the program, but that's all.

well, I found a workaround - by forcing the sketch to always have screen proportions (rather than basing on height width) - when run in full screen OSX scales to fit, so a sketch larger than the screen is scaled down. Screen proportions removes the boundary.