Added prism.js theme + demo

Hi Pasquale!

I just added the `css` file to my forked repo, and also added a demo folder (same files as [my codepen](http://codepen.io/flovan/full/culDr)) just in case you want to include that as well.
I wanted to update the `README.md` as well but I don't know how and if you want to add a preview image, so I'll leave that up to you!

Thanks again for the great theme, I really love using it!