chore: Refine layout of treeview for One Theme

[at-susie]

Description

This PR updates the space and layout of the tree-view for One Theme only. Specifically, we tighten the visual gap between the expand toggle and tree item content by shrinking the toggle column from 28px to 24px, matching the smaller (12px x-small) toggle icon used in One Theme. Visual refresh (16px normal icon, 28px column) is unchanged.

Also re-centers the vertical connector line on the narrower One Theme column.

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 97.66%. Comparing base (6de850b) to head (793ca84).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4964   +/-   ##
=======================================
  Coverage   97.66%   97.66%           
=======================================
  Files         959      959           
  Lines       31342    31342           
  Branches    11578    11577    -1     
=======================================
  Hits        30610    30610           
- Misses        686      725   +39     
+ Partials       46        7   -39     

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