fix: Change theme dynamically when system preferences change

[holloway]

Although the theme was correctly applied on page load we weren't monitoring OS theme preference changes that occur afterwards. This watches for these changes and recalculates the theme

Fixes #7739

[holloway]

fyi @rjsparks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.84%. Comparing base (c7f6bde) to head (162c062).
Report is 82 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7956      +/-   ##
==========================================
+ Coverage   88.78%   88.84%   +0.05%     
==========================================
  Files         296      304       +8     
  Lines       41320    41525     +205     
==========================================
+ Hits        36687    36891     +204     
- Misses       4633     4634       +1     

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