Thank you for your post.
The Theme files are in the same place as they were for FileMaker Pro 9.
On Windows, FileMaker Pro Advanced \ Extensions \ English \ Themes
On Macintosh, FFileMaker Pro Advanced -> FileMaker Pro Advanced.app -> Contents -> Resources -> English.lproj -> Themes
Let me know if you need clarification.
You are almost correct !
What you and all the Filemaker Pro Advanced documentation miss out, which is vital, is that on Macintosh you must right click on the application in order to see the package contents. On Filemaker Pro Developer 6 the theme files were in a directory.
I found the answer on another forum, but thanks for taking the time to try and help.