Extend themes guide

[MichaelCurrin]

List them and use screenshots or link to demos or page with screenshots

Content:

In this case I've chosen a Read the Docs theme so it looks like a Python project hosted on Read the Docs, without using Sphynx. Plus if I decide I want a GitBook or Material or default theme, its only a config option away. For Jekyll, there's a lot more you might have to change to switch themes