Make background around canvas configurable

[novhack]

Currently the background around the canvas is set by a theme. I think it would be nice to be able to set the color in preferences.
There could be a checkbox "use canvas background color from theme" to set the color back to default theme color (it could be checked by default).

[Variable-ind]

I just made it, you could check it out here: https://github.com/Variable-ind/Pixelorama/tree/Make_background_around_canvas

background_color.mp4

[Variable-ind]

Also, i was thinking if this could be moved to "Discussions"

[OverloadedOrama]

Implemented in 66857f5. I placed the option under Canvas because it seems more relevant there, but we could place it under Interface is desired.