Support light and dark themes

[jacob314]

We should start supporting both light and dark themes for all our UI sooner rather than later so it is less work to refactor later.
VS Code and Android Studio users tend to use a dark theme so having a dark theme will make the devtool tool feel more consistent.

[jacob314]

https://github.com/StylishThemes/GitHub-Dark
looks like a reasonable place to start.

[jacob314]

It would be fine to only support switching themes on app start. For example with a query parameter.
e.g.
?theme=dark

[jacob314]

I'll take a stab at adding the dark theme as an option.

[kenzieschmoll]

Dark theme being tracked here: #418.