fix: add background to modal body to fix overflow

[rpenido]

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[openedx-webhooks]

Thanks for the pull request, @rpenido!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.27%. Comparing base (020ae21) to head (8cd7200).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3238   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         249      249           
  Lines        4401     4401           
  Branches     1037     1037           
=======================================
  Hits         4105     4105           
- Misses        290      293    +3     
+ Partials        6        3    -3     

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