<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
This PR ensures that Light Switch detects changes to app/system theme
from Windows Settings. This PR also introduces new behavior where
switching the schedule will cause an instant update to the theme if
necessary.
<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist
- [x] Closes: #42878
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx
