It will happen with every theme you try.

7.9 is really buggy and definately not the most stable version of PHP-Nuke..it is HIGHLY recommended you revert back to version 7.6 patched with 3.2 patch from Chatserv.

There are some manual changes you must make to the themes you want to use in order to get them to work right.

THe 3.2 patch package has a README file that explains this