revert: Removes link underline color tokens from #4621

[pan-kot]

Description

A partial revert of #4621.

Reason: the currentValue is not accepted by theming, but an explicit value changes link styles when links are styled or focused. Need to re-evaluate the decision for themeable link underline tokens.

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 97.51%. Comparing base (560af5d) to head (e43d581).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4635   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files         948      948           
  Lines       30346    30346           
  Branches    11067    11067           
=======================================
  Hits        29593    29593           
  Misses        706      706           
  Partials       47       47           

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