[Darkmatter] Create new darkmatter theme

[rukmini-bose]

Closes https://github.com/nasa/openmct/issues/7681

Describe your changes:

• Create new theme plugin for darkmatter
• Create new .scss file with initial stylistic changes. More changes to come in a separate PR

All Submissions:

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ ] Is this a notable change that will require a special callout in the release notes? For example, will this break compatibility with existing APIs or projects that consume these plugins?

Author Checklist

• [x] Changes address original issue?
• [ ] Tests included and/or updated with changes?
• [ ] Has this been smoke tested?
• [x] Have you associated this PR with a type: label? Note: this is not necessarily the same as the original issue.
• [ ] Have you associated a milestone with this PR? Note: leave blank if unsure.
• [ ] Testing instructions included in associated issue OR is this a dependency/testcase change?

Reviewer Checklist

• [ ] Changes appear to address issue?
• [ ] Reviewer has tested changes by following the provided instructions?
• [ ] Changes appear not to be breaking changes?
• [ ] Appropriate automated tests included?
• [ ] Code style and in-line documentation are appropriate?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 56.40%. Comparing base (fb0d74e) to head (4bc0cea).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7682      +/-   ##
==========================================
- Coverage   56.72%   56.40%   -0.33%     
==========================================
  Files         672      673       +1     
  Lines       27182    27185       +3     
  Branches     2635     2635              
==========================================
- Hits        15420    15333      -87     
- Misses      11432    11522      +90     
  Partials      330      330              

Flag	Coverage Δ
e2e-full	23.55% <33.33%> (-18.02%)	:arrow_down:
e2e-stable	59.97% <33.33%> (-0.03%)	:arrow_down:
unit	49.21% <33.33%> (-0.04%)	:arrow_down:

Files	Coverage Δ
src/plugins/plugins.js	100.00% <100.00%> (ø)
src/plugins/themes/espresso.js	100.00% <ø> (ø)
src/plugins/themes/snow.js	0.00% <ø> (ø)
src/plugins/themes/darkmatter.js	0.00% <0.00%> (ø)

... and 13 files with indirect coverage changes

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[rukmini-bose]

Testing Instructions: https://github.com/nasa/openmct/issues/7681

[ozyx]

LGTM

wowwwww