fix(styling): remove different bg-color on unorderable column

[ghiscoding]

It looks a little weird that the locked (unorderable) columns have different background-color especially when our grid doesn't have any borders around the headers. So I think it's better to remove it and fix the styling discrepancy by using the same bg-color for all column headers but keeping the SASS variable in case the user still want to change it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (44f6c08) 99.7% compared to head (e2693be) 99.7%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1358   +/-   ##
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  Coverage    99.7%   99.7%           
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  Files         199     199           
  Lines       21576   21576           
  Branches     7202    7202           
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  Hits        21496   21496           
  Misses         73      73           
  Partials        7       7           

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