chore: Update custom dev pages to use light-dark method

[teodoranemes]

Description

Refactored the custom style dev pages to use the light-dark() method for mode switching.

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (71f259e) to head (1868ade).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3896   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files         854      854           
  Lines       24983    24983           
  Branches     8803     8803           
=======================================
  Hits        24276    24276           
  Misses        659      659           
  Partials       48       48           

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