Isolating Cortana's theme CSS from application CSS

[justinhelmer]

When building a style guide, you might want your style guide to use Cortana, but your app CSS is completely different. In this case, the Cortana CSS and your application CSS will conflict, causing both the Cortana theme and your rendered example output to be impacted.

What you really want is the theme CSS and application CSS to be completely isolated from one another. But how?

Here is my solution; after some refining, it seemed valuable enough to share: (originally posted to the hologram-github-theme issue queue)