[Backport] fix: Update custom SSO buttons to use brand colors

[ihor-romaniuk]

Description

This pull request is a backport from the master branch #1165.
The main goal is to make the buttons reflected to the primary paragon color by using the variable provided by the Paragon library.

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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@9f503eb). Click here to learn what that means.

Additional details and impacted files

@@                      Coverage Diff                      @@
##             open-release/quince.master    #1208   +/-   ##
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  Branches                              ?      635           
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  Hits                                  ?     2000           
  Misses                                ?      286           
  Partials                              ?        9           

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[mphilbrick211]

Hi @openedx/2u-vanguards! This should be all set, as the master has been merged. Thanks!

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@ihor-romaniuk 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.