[Settings] React on OS theme change fix

[stefansjfw]

Summary of the Pull Request

Register theme listener properly to react to OS theme change immediately.

PR Checklist

• Closes: [Settings]Takes some time to adapt to theme change #26791
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

• Open Settings app
• Ensure Theme setting is set to "Windows default"
• Change OS Theme settings and observe that Settings app changed it's theme properly
• Ensure that different Theme settings work as expected

[Jay-o-Way]

Just to check: Are HighContrast themes independent of the light/dark bits?

[Jay-o-Way]

Works nice, thanks!

[niels9001]

Tested all variants (inc. different High Contrast themes and moving between HC to light/dark and vice versa). Works great :)!!

[jaimecbernardo]

LGTM! Working so much better and much simpler now. Thank you!