Advanced documentation: setting up the css

[badlydrawnrob]

Easy way:

@include main.css and override --css-variables or plain css.

Harder way:

@include the modules/*.less and partials/*.less files and override where needed

• Pandoc Skylighting theme (and some complaints about it)
• Css files aren't completely clear for extending anki#67
• Potentially add more colours and themes anki#52
• Explore font stacks anki#12
• Overriding --css-variables in Anki anki#78 (simpler explanation)