Refactor Highlight CSS loading to avoid css-loader expectation

[kylesuss]

An interesting thing happened when I was trying to import this design system into another app -- the CSS file in the Highlight component was throwing errors as if it could not be loaded properly. I've realized that there is not an issue w/ this in Storybook as they have their own CSS loader and so does Gatsby (which is why it didn't come up during the learnstorybook.com development).

Therefore, I have decided to just copy over the prism CSS theme rather than try to import the CSS file for a few reasons:

1. The build process in this repo does not utilize webpack, so it would be more difficult to add that in addition to the css-loader
2. There did not seem to be a babel equivalent to webpack's css-loader that I could use instead (we build the design system w/ babel and not webpack)
3. It seems like we should not assume that the consumers of this design system should have to set up the css-loader

[kylesuss]

🚀 PR was released in v0.0.27 🚀