Use CSS-in-JavaScript

[lencioni]

We don't have much CSS, but it would still be nice to have the benefits of CSS-in-JavaScript and be free of the drawbacks of external CSS files.

I think there are two good options that we should consider right now.

1. react-with-styles
2. styled-components

And I think we should go with styled-components because it is simpler to use and should handle everything that we need it to.

[trotzig]

Are you sure we shouldn't use the project you've been working on?

We should pair this with setting up happo diffs in Travis CI for our components.