Fix switching theme

[stefanschaller]

:scroll: Description

If previously didn't work when switching the theme, since a lot of widgets didn't receive the correct brightness after the theme was switched.

:bulb: Motivation and Context

It's now working fine.

:green_heart: How did you test it?

Yes, fully tested in the example app and in our app, with fetci

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes -> Not a new feature
• [x] I updated the docs if needed
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps