Available Themes Doc

[Brennan Toles]

https://github.com/jxnblk/mdx-deck/blob/master/docs/themes.md

All of the examples are missing the @ for the mdx-deck scope.

[Giovanni Maria Cusaro]

@Btoles and others things 🤔 #677 (comment)

[Christopher Peterson]

"other things" aside 🙂 I made a pull request for the themes docs #729