fix dark theme scroll

[ahuang11]

Closes #7600

Don't fully understand CSS, but adding ::-webkit-scrollbar seems to make it properly use the theme's scrollbar colors

After:

Before:

With scrollbar:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.12%. Comparing base (400cc44) to head (711e96b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7601   +/-   ##
=======================================
  Coverage   82.11%   82.12%           
=======================================
  Files         340      340           
  Lines       51477    51477           
=======================================
+ Hits        42271    42274    +3     
+ Misses       9206     9203    -3     

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