With the addition of EPUB support to doc-view I took a shot at
adding support for mutool's user stylesheet feature, which allows the
user to control the layout of the EPUB with custom CSS rules.

I'm attaching the patch here if anyone is interested.  This is my first
attempt at contributing to Emacs, so I'm sure there is something I got
wrong/did sub-optimally.  Any criticism/suggestions for improvement are
welcome.