chore: Replace background with a proper token

[at-susie]

Description

This PR replaces the hardcoded background color for the code view component with the proper token that we introduce in cloudscape-design/components#4638.

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.

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 100.00%. Comparing base (d31e724) to head (9e472ad).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #145   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines           78        78           
  Branches        21        21           
=========================================
  Hits            78        78           

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