g1smd - 2:56 pm on Mar 5, 2012 (gmt 0)
After Xenu scans the website via HTTP (the site therefore needs to be running on a HTTP server such as Apache) it then asks for the FTP credentials so it can look in all the folders to find any files that were not accessed during the HTTP scan - those are the unused files.
I have no idea if Xenu looks for files mentioned in style sheets. I have never considered that possibility. I would hope that it does. It is quite easy to test whether it does or not.