chore: POC - define dnd border with CSS var

[pan-kot]

Description

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

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 96.40%. Comparing base (b3e4bf7) to head (e0a577c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3154      +/-   ##
==========================================
- Coverage   96.40%   96.40%   -0.01%     
==========================================
  Files         784      784              
  Lines       22136    22133       -3     
  Branches     7190     7532     +342     
==========================================
- Hits        21341    21338       -3     
+ Misses        788      743      -45     
- Partials        7       52      +45     

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