fix(observability-callout): update styles for improved contrast

[RemiBonnet]

Summary

Issue:

Screenshots / Recordings

Testing

• Changes tested locally in the relevant Console's pages and Storybooks
• yarn test or yarn test -u (if you need to regenerate snapshots)
• yarn format
• yarn lint

PR Checklist

• I followed naming, styling, and TypeScript rules (see .cursor/rules)
• I performed a self-review (diff inspected, dead code removed)
• I titled the PR using Conventional Commits with a scope when possible (e.g. feat(service): add new Terraform service) - required for semantic-release
• I only kept necessary comments, written in English (watch for useless AI comments)
• I involved a designer to validate UI changes if I am not a designer
• I covered new business logic with tests (unit)
• I confirmed CI is green (Codecov red can be accepted)
• I reviewed and executed locally any AI-assisted code

[RemiBonnet]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (new-navigation@18297bb). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             new-navigation    #2446   +/-   ##
=================================================
  Coverage                  ?   45.32%           
=================================================
  Files                     ?      543           
  Lines                     ?    12318           
  Branches                  ?     3554           
=================================================
  Hits                      ?     5583           
  Misses                    ?     5783           
  Partials                  ?      952           

Flag	Coverage Δ
unittests	45.32% <ø> (?)

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