👌 IMPROVE: Move CSS colors to variables

[chrisjsewell]

for easier overriding

[codecov]

Codecov Report

Base: 81.81% // Head: 81.77% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (175761a) compared to base (5bf2805).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
- Coverage   81.81%   81.77%   -0.04%     
==========================================
  Files          29       29              
  Lines        2639     2639              
==========================================
- Hits         2159     2158       -1     
- Misses        480      481       +1     

Flag	Coverage Δ
pytests	81.77% <ø> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_nb/core/render.py	85.34% <0.00%> (-0.22%)	⬇️

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