fix: remove `$primary-color` & fix more styling issues for ms-select

[ghiscoding]

• removing SASS $primary-color (and keep only $slick-primary-color) since it could cause certain variables to not be overriden properly. I previously kept the $primary-color to make it easier for Bootstrap users but this is causing issues with the new SASS @use syntax
• also more styling fixes to align with the rest of Bootstrap input form controls, note though that the fixes doesn't only apply to Bootstrap but other frameworks as well

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.8%. Comparing base (2b78877) to head (2841dbc). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1724 +/- ## ====================================== Coverage 99.8% 99.8% ====================================== Files 187 187 Lines 30697 30697 Branches 9763 9763 ====================================== Hits 30608 30608 Misses 89 89 ```

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