Following my patch from a few months ago which added the
`doc-view-mutool-user-stylesheet' defcustom to Doc-View making use of
mutools ability to apply a user specified CSS file when converting a
EPUB file.  Here is a patch to reconvert existing EPUB buffers when the
value of that variable (renamed to `doc-view-epub-user-stylesheet' as
discussed after my last patch) is changed, or the specified file is
changed, therefore applying the new CSS immediately.