feat: add isThemeActive helper

[mxschll]

Issue #, if available: aQokArCTx9ab

Description of changes:

Adds isThemeActive helper that checks both class presence on the document body and window-level feature flags, with class presence taking precedence (like it's being done with isVisualRefresh).

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (3ff2844) to head (adc8b41).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   98.99%   99.00%           
=======================================
  Files          44       44           
  Lines        1293     1304   +11     
  Branches      337      338    +1     
=======================================
+ Hits         1280     1291   +11     
  Misses         13       13           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.