fix: set header bg to transparent if root bg is provided

[mxschll]

Description

Changes done:

• Fix for AWSUI-61442
• Introduces unit tests

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

How has this been tested?

Visual regression test.

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 97.12%. Comparing base (83500f3) to head (4dd986c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4029      +/-   ##
==========================================
- Coverage   97.12%   97.12%   -0.01%     
==========================================
  Files         864      864              
  Lines       25330    25326       -4     
  Branches     9139     9139              
==========================================
- Hits        24602    24598       -4     
  Misses        722      722              
  Partials        6        6              

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