[KevinH]

That should not happen.

Using AddExisting on an xhtml file has always pulled in any properly linked images and stylesheets automatically.

So did your backmatter xhtml already have a link to a stylesheet with the exact same name? Did that pulled in stylesheet file actually exist at the relative location pointed to in the backmatter file?

Either way, please attach as zips to this thread the stylesheet file and backmatter .xhtml (scramble the xhtml and cut the stylesheet down to just one class if needed) but keep all file names exactly as they were and include any img files or stylesheets that had proper relative links to your backmatter code.

That way we can try to recreate what you are seeing.

And fwiw, loading a prebuilt official template eoub with all base pieces already in place is a much faster way to get started. Epub templates can be created for both epub2 and epub3 style epubs and co-exist in your Sigil Prefs folder happily.