Evergreen builds do not transform CSS variables

[rorticus]

Update to disable css custom properties when not doing a legacy build.

Before:

--my-color: blue;

.myClass {
    color: blue;
    color: var(--my-color);
}

Now,

--my-color: blue;

.myClass {
    color: var(--my-color);
}

[codecov[bot]]

Codecov Report

Merging #37 into master will decrease coverage by 0.19%. The diff coverage is 0%.

@@            Coverage Diff            @@
##           master      #37     +/-   ##
=========================================
- Coverage   70.83%   70.63%   -0.2%     
=========================================
  Files           8        8             
  Lines         360      361      +1     
  Branches       63       64      +1     
=========================================
  Hits          255      255             
- Misses        105      106      +1

Impacted Files	Coverage Δ
src/base.config.ts	33.87% <0%> (-0.56%)	:arrow_down:

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[agubler]

Closing for now (very outdated), we might address this in a subsequent change