I don't think thats possible.
3 years later (almost to the day) and I'd like to ask the same question of Fireworks CS5...
How do you change the color of the workspace background (outside of the canvas) ?
I want my canvas to be the same color as the outside of the canvas, but this makes it impossible to tell if I have positioned everything in the canvas properly.
It seems like such a simple preference setting, but I can't find it anywhere.
It will save me continually changing canvas color to see what's going on then changing back before exporting.
PS - I don't want to know about workarounds or plugins or suggestion of other ways to do what I want - If it can't be done simply then where do I request it as a new feature ?
Have requested it just now.