Whenever I make a new canvas it creates a white background. I'd like for new canvas to have a black background.

It's not possible at the moment without going in and changing the C++ source. If you have no problem with that, take a look at line 455 of milton.cc as of the moment of this writing.

Milton's development has slowed down quite a bit in the past year or so, but this is one of those features that are going to be in there eventually. I just don't know when!