refactor: Clean up golden-layout css

[dsmmcken]

We maintain our golden-layout package, bake our theme customizations directly into the dark theme. Eliminates some css. Used e2e testing to prevent visual regressions, the two changes showing are intentional.

Had to exclude the ThemeExport from the karma tests, it didn't like the scss import.

[codecov]

Codecov Report

Merging #1322 (7bc888a) into main (485c7cc) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1322      +/-   ##
==========================================
- Coverage   45.95%   45.94%   -0.01%     
==========================================
  Files         493      493              
  Lines       34404    34410       +6     
  Branches     8580     8581       +1     
==========================================
  Hits        15811    15811              
- Misses      18542    18548       +6     
  Partials       51       51              

Flag	Coverage Δ
unit	45.94% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
packages/dashboard/src/PanelPlaceholder.tsx	40.00% <ø> (ø)
packages/dashboard/src/layout/LayoutUtils.ts	13.94% <0.00%> (ø)

... and 1 file with indirect coverage changes