All of the platform have dark modes now, so we need to support them, even though they're pretty much always terrible.

At minimum, we need to respect the user’s light/dark choices.

It would also be good if we could allow the user to set a preference - default to “auto” (respect system settings), but allow user to force light or dark. This might not yet be possible with wxPython; need to investigate. Potentially also allow different settings for windowed vs. full screen.