Thanks for the suggestion - have not tried System Restore. Culled most of my earlier restore points, so now the earliest ones I have available are all dated 6/24, after the problem started. So that option is out.
Strange thing is I get no error message. It just will not save to the .themepack file format. I've gone into the registry to see if I could find anything that might help. No luck. I tried uninstalling several programs that were installed around the date the problem started, also no luck.
Unless someone has managed to resolve this issue, it appears my only option would be a reinstall of Windows 7. Not going there as have my computer set up the way I like it.
So I'm left with continuing to use Desktop Architect theme management program to "apply" my custom settings. Compared to some of the issues others are dealing with when it comes to themes, that's not so great a problem. It's just that I absolutely hate having something not working properly and not be able to "fix it". Usually I can without help from others.
Would welcome any ideas other forum members may have. Would still like to be able to save my custom themes so can install them from a .themepack file. Have found that seems to be the only way to prevent the desktop image from changing to the one for the current theme each time you simply "save" a theme.