Make it easier for users to add their own CSS rules

[bitprophet]

As far as I can tell from Sphinx's own docs, users can inject static files by themselves by throwing _t files into their html_static_path (defaults to empty list but of course configurable in users' conf.py).

That gives them the ability to insert CSS files, and thus, lets them override built-in styles without us needing to expose literally everything as a config option (just...almost everything).

The missing piece, then, is that our templates need to slap another <link> tag into the extrahead block so that the resulting HTML tries loading such an auxiliary CSS file. I don't see the basic theme (which we inherit from) doing this as-is, so it does fall to us. Should be trivial.

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This would incidentally solve #76.