feat: unification of the appearance of buttons

[ihor-romaniuk]

Description

This pull request contains recommendations to improve the display of button blocks for oauth2 providers.
The main goal is to make the buttons appear uniform so that it looks more aesthetically pleasing and consistent.
Also, removing SCSS-specific variables and using the variables provided by the Paragon library.

Related Pull Requests

PR to the open-release/palm.master branch: #1090
PR to the master branch: #1092

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (open-release/quince.master@60e2119). Click here to learn what that means.

Additional details and impacted files

@@                      Coverage Diff                      @@
##             open-release/quince.master    #1091   +/-   ##
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  Hits                                  ?     1970           
  Misses                                ?      286           
  Partials                              ?        9           

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[mphilbrick211]

Hi @openedx/vanguards! This is ready for review / merge once the master is all set.

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