Decide on CSS tool and convert App.css

[markpalfreeman]

• Sass or styled-components?

The syntax of either one will be the same, but there are a couple tradeoffs either way. Sass is a little more familiar and leaves your markup/components alone (more traditional). However, React plays really well with new-fangled CSS-in-JS libraries like styled-components, and it enforces "put your styles with your markup" by design.

We need to make a decision on which way to go, then:

• implement the tooling (see create-react-app docs)
• convert App.css to use the new way