chore: Extract common visual theme code into helper function

[michaeldowseza]

Description

Ports over helper function from cloudscape-design/components#3355

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

How has this been tested?

• Copied over unit tests

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 93.66%. Comparing base (7468aa5) to head (ddbd104).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
+ Coverage   93.59%   93.66%   +0.06%     
==========================================
  Files          61       68       +7     
  Lines        4312     3124    -1188     
  Branches      525      670     +145     
==========================================
- Hits         4036     2926    -1110     
+ Misses        276      194      -82     
- Partials        0        4       +4     

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