Material UI Light + Dark Mode

[mikhael28]

We recently upgraded our Material UI from v3 (lol) to the latest and greatest, v5 - thank you @vmcodes - shortly after that, @jayeclark went through the process of adding polish to a lot of UI, and changing the UI to an overall dark mode aesthetic.

Moving forward, we need to add a Dark/Light mode toggle and update the base styling of MUI to accommodate global stying changes. Ideally, this toggle would live in the LeftNav or somewhere else obvious and universal.

When going for this feature, it will require clicking through the entire application to check that their changes have been accepted universally (or, at least 80% for this intro PR - lots of small tweaks can be made later).