Make it clearer how to customize the look and feel of the site

[choldgraf]

@emdupre mentioned that the current guide isn't super clear about how people can modify the jupyter book content - this should be improved, maybe with a nicer set of sub-sections and some examples for particular looks (in particular, the distill.pub style is really nice)

[emdupre]

Yes, I think documenting this functionality would be huge ! In case anyone else is also looking at the distill.pub style, the CSS files are available here.