It is the HTML output file that refers to the CSS file, and it looks only in the current directory.  I.e., the feature is not really fully-baked, and it is entirely upstream's (that means my) fault.

As far as packaging it, for now the best thing you can do is probably to stick it in a documentation directory somewhere.  But if you have any suggestions for how to organize this better in the upstream project, I'm all :ear:s.