OS Dark theme compatibility

[marc-vdm]

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation
  • For in-code documentation, please follow the numpy style guide.
  • For user documentation, please update the wiki in
    ./activity_browser/docs/wiki
• Update tests.
• Link this PR to related issues by using closing keywords.

If you have write access (otherwise a maintainer will do this for you):

• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 54.266% (+0.03%) from 54.233%
when pulling c02ad24 on marc-vdm:dark_theme_settings
into 6ebca06 on LCA-ActivityBrowser:main.

[mrvisscher]

Is there a way to not have to reload the AB? Like setting the style_item singleton