Link color matches default background

[elwayman02]

If you follow the quickstart guide and set up your 404 route, you'll notice this immediately:

It's only visible if you highlight the text, since the colors are exactly the same.

[elwayman02]

Interestingly, the 404 page for this project's own docs site has customized CSS to fix this problem: https://ember-learn.github.io/ember-cli-addon-docs/foo

Instead of having custom CSS, we should do our best to have no special configuration for this demo site whatsoever. We need to know that the styles provided in this project work out of the box for the majority of users.