Rename theme variables to match industry practice

[yurishkuro]

Which problem is this PR solving?

• Part of [Feature]: Add dark theme support to Jaeger-UI #1911

Description of the changes

• Rename what minimum number of vars is used to follow semantic naming practice

How was this change tested?

• added css linter to verify that vars are not missing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.92%. Comparing base (b03affc) to head (d2c0d04).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3174   +/-   ##
=======================================
  Coverage   97.92%   97.92%           
=======================================
  Files         260      260           
  Lines        8162     8162           
  Branches     2122     2151   +29     
=======================================
  Hits         7993     7993           
  Misses        169      169           

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