[Darkmatter] Add New Theme Plugin

[rukmini-bose]

Is your feature request related to a problem? Please describe.
What's the matter? Darkmatter.

Describe the solution you'd like
I want to add a new theme plugin and .scss file to fix the matter that's bringing us darkness.
Jokes aside, let's merge this initial theme branch into master. Things to look out for– any lines of code that may affect the functionality and the look & feel of the other available Open MCT themes (Espresso and Snow).

Describe alternatives you've considered

Additional context
Add any other context or screenshots about the feature request here.

[rukmini-bose]

Testing Instructions:

1. Open Open MCT.
2. In the console, type in openmct.install(openmct.plugins.DarkmatterTheme()); to change the theme to Darkmatter.
3. Verify that the look & feel for Open MCT changes.