[dev] Do not use deprecated Sass @import in dummy app

[tvdeyen]

What is this pull request for?

It works but also clutters the dummy app log

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.37%. Comparing base (87fc9cc) to head (8c7b124).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3574   +/-   ##
=======================================
  Coverage   97.37%   97.37%           
=======================================
  Files         312      312           
  Lines        8087     8087           
=======================================
  Hits         7875     7875           
  Misses        212      212           

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