fix: button hover background color

[KristinAoki]

Description

This PR updates the hover background for the top navigation buttons that are shown when the left side navigation is enabled. The hover background is updated to match the hover background of other existing IconButton components, see DiscussionNotificationTrigger.jsx, on the page.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.09%. Comparing base (226c4cc) to head (207ede8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1653   +/-   ##
=======================================
  Coverage   90.09%   90.09%           
=======================================
  Files         338      338           
  Lines        5784     5784           
  Branches     1370     1407   +37     
=======================================
  Hits         5211     5211           
+ Misses        557      556    -1     
- Partials       16       17    +1     

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