Thank you Sujit for links. The solution appears to be editing of the master rules. It seems the Default Portal Desktop might be renamed the Classic Portal Desktop. The default master rules make the AFPDesktop the default for all users in 7.4. I can likely change the master rules to default to the Classic Desktop for all users and the problem should be solved. No issues applying custom theme to the classic desktop.
Now just looking for documentation and any other related differences.