Argument `theme` of `raw.plot()` or config key does not work on macOS

As spotted following the discussion here https://github.com/mne-tools/mne-qt-browser/issues/83 the argument `theme` does not change the theme on either my macOS or Windows computer.

Sample code:

```
import mne


folder = mne.datasets.sample.data_path() / 'MEG' / 'sample'
raw = mne.io.read_raw(folder / 'sample_audvis_filt-0-40_raw.fif', preload=True)
raw.plot(theme='dark')
```