[core] Fix missing borders for select elements

[ricoberger]

After the last dependency update, the select elements had no border,
when the React UI was build for production via "yarn build". To fix this
we import the "index.css" file with the fix after all other components
in the "index.tsx" file for out app.

• I added a CHANGELOG entry for this change.
• I adjusted the corresponding documentation for this change.
• I adjusted the values.yaml file and the documentation for all values.

[codecov-commenter]

Codecov Report

Merging #303 (890fce2) into main (dfecbf8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #303   +/-   ##
=======================================
  Coverage   44.73%   44.73%           
=======================================
  Files          96       96           
  Lines        8116     8116           
=======================================
  Hits         3631     3631           
  Misses       4453     4453           
  Partials       32       32           

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