chore: Allow borderItemWidth and colorBorderDropdownItemDefault to be themed

[avinashbot]

Description

Follow up from #4986. The core update theme requires certain tokens to be themeable, and these two fell through the cracks. The public: false, themeable: true is on purpose; we have a number of tokens already like that and like the other, this lets us make it "soft"-public first.

Related links, issue #, if available: n/a

How has this been tested?

Tested locally, theming works on these tokens on the scoped-theme dev page without throwing a dev warning.

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 97.66%. Comparing base (4b4d1f4) to head (cd04e18).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5012   +/-   ##
=======================================
  Coverage   97.66%   97.66%           
=======================================
  Files         960      960           
  Lines       31368    31368           
  Branches    11593    11593           
=======================================
  Hits        30637    30637           
  Misses        685      685           
  Partials       46       46           

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