Remove duplicate css vars

[agubler]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier
• Unit or Functional tests are included in the PR

Description:

Use OptimizeCssAssetsPlugin with cssnano to remove duplicates from the css assets.

[codecov]

Codecov Report

Merging #219 into v4 will decrease coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##              v4     #219      +/-   ##
=========================================
- Coverage   52.7%   52.67%   -0.03%     
=========================================
  Files         10       10              
  Lines        573      579       +6     
  Branches     140      141       +1     
=========================================
+ Hits         302      305       +3     
- Misses       271      274       +3

Impacted Files	Coverage Δ
src/base.config.ts	23.49% <50%> (+0.99%)	⬆️

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