I've been playing around with using contentStyleFile in page-mod: https://addons.mozilla.org/en-US/developers/docs/sdk/latest/packages/addon-kit/page-mod.html#Styling_web_pages What I found is that if you have media in the style sheets added that are using relative paths, e.g. background: rgb(249, 249, 249) url('../../img/mozorg-gradient.png') repeat-x top center; ... they are not found. What scope are files loaded via contentStyleFile in? How can I include images and other media in these stylesheets? It might get painful to add custom paths to each, especially in the case of stylesheets of 3rd party libraries for example.
This bug should be fixed now, by bug 837494. Feel free to re-open it if the behavior persist.