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 97.16%. Comparing base (a4a22ce) to head (f8e628b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3896   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files         850      850           
  Lines       24776    24776           
  Branches     8732     8731    -1     
=======================================
  Hits        24073    24073           
- Misses        654      696   +42     
+ Partials       49        7   -42     

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