[Refactor:System] Remove Unused Colors from colors.css

[JManion32]

In colors.css, there were 14 colors not being used anywhere in the project. I deleted them and verified that Submitty's UI in light mode and dark mode remained unchanged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 22.09%. Comparing base (c4dab15) to head (3f71191).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #11433   +/-   ##
=========================================
  Coverage     22.09%   22.09%           
  Complexity     8835     8835           
=========================================
  Files           246      246           
  Lines         31805    31805           
  Branches         79       79           
=========================================
  Hits           7026     7026           
  Misses        24703    24703           
  Partials         76       76           

Flag	Coverage Δ
autograder	21.56% <ø> (ø)
js	26.85% <ø> (ø)
migrator	100.00% <ø> (ø)
php	19.09% <ø> (ø)
python_submitty_utils	80.08% <ø> (ø)
submitty_daemon_jobs	90.94% <ø> (ø)

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[hinyan17]

Verified that the colors were not used anywhere else in Submitty. UI looks the same in light and dark mode too.

[max1dyer]

I verified all colors removed are not used in any other files. The site looks the same in light and dark mode. Some hex color codes are used for multiple color variables but since the other colors are currently being used that shouldn't matter.