We need a "large fonts" and "small fonts" theme to ship with Discourse

[sam (Sam Saffron)]

@Mittineague

For whatever reason, I was thinking there was a way to pull in the CSS similar to how plugins can be added to app.yml

Themes are way more flexible than plugins, you simply check it in to GitHub and then anyone can import it. For the large font one we will seed an extra theme in the theme file.