Toolbar and statusbar widgets are not in dark mode when my system is in dark mode

[hoechenberger]

So this is what it looks like on macOS, makes my eyes bleed:

[larsoner]

I fixed this in #66, I suspect some PR to MNE-Python after mne-tools/mne-python#10414 (which was needed as well) broke this actually :(

cc @GuillaumeFavelier this is probably related to some of the recent theme refactoring(s). We have to be careful about when we set stylesheets and such.

@hoechenberger can you 1) check that switching to MNE-Python [6e2c52c](https://github.com/mne-tools/mne-python/commit/6e2c52c592be02de3c798c98b21cfdc1d6e264ad) works, and then if so 2) git bisect to figure out which PR broke things? I'm guessing it might be mne-tools/mne-python@4301092 / mne-tools/mne-python#10456 because IIRC it messed with themes most recently, but it's very possible I'm wrong!

[hoechenberger]

In fact I didn't even check my version of MNE-Python, I will check to be running the latest version tomorrow to see if the problem persists. Maybe there's nothing wrong after all …