Fix dark mode appearance

[Supereg]

Fix dark mode appearance

♻️ Current situation & Problem

This PR fixes appearance in dark mode. Due to refactorings to lists in #18, we know have to adapt to different backgrounds (e.g., placed directly into a view, placed into a grouped background view, like a List cell).
This was primarily an issue as we used a background color instead of a fill color (see tertiarySystemFill being recommended for buttons and compare to secondarySystemBackground which we previously used).

⚙️ Release Notes

• Fixes appearance in light background when appearing in Lists.

📚 Documentation

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✅ Testing

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📝 Code of Conduct & Contributing Guidelines

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• I agree to follow the Code of Conduct and Contributing Guidelines.

[codecov]

Codecov Report

Merging #19 (079d91e) into main (bd38bd7) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #19   +/-   ##
=======================================
  Coverage   81.23%   81.23%           
=======================================
  Files           4        4           
  Lines         309      309           
=======================================
  Hits          251      251           
  Misses         58       58           

Files	Coverage Δ
...urces/SpeziContact/Contact Views/ContactView.swift	82.36% <100.00%> (ø)

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[PSchmiedmayer]

Nice, thank you for the fix @Supereg and thanks for taking over the review @philippzagar 🚀