[KevinH]

Also if you want to launch an external CSS editor, why can't you just use Open With. For epubs with only few stylesheets that should work just fine.

For editing lots of CSS stylesheets at the same time, you could use the Launch External XHTML and pass the entire opf to your application, but whatever is launched would need to be smart enough to parse the opf and extract and open each css stylesheet.

FWIW, I am still thinking about how to identify and merge common selectors across stylesheets but I am leaning toward doing this in a plugin as most epubs have less than a handfull of stylesheets.