"Theme" support

[adamghill]

I'm thinking this could just be a defined directory structure for templates + static assets?

[adamghill]

It would also be nice to have a default "theme" so users can just create markdown files and things look good "out of the box".

https://watercss.kognise.dev/ is used now in the base template, but https://simplecss.org/demo also looks nice.

[adamghill]

https://github.com/bigskysoftware/missing is another option for a base css class.