Draft: Add and use a Vuetify theme

This should hopefully be the start of a package/tool for theming the Trame applications in a way that minimizes the need for front-end knowledge by application developers.

I think it's probably better to have the theme code exist in a separate repo, but I'm not sure where that would live. Regardless, I think I'd like for you all to review and give feedback on this before we take this too far.

Closes #17 (closed)