I've prepared a simple and useful script for you, that will get you rid of problems with bad installed themes.

If you set a bad default theme for your page, whole page will not
work and error messages will be displayed. In this case, this script
sets the default theme of installed system a makes it fully functional
with this theme, which can be changed in administration section.

It's for:

United-Nuke 4.1 series

United-Nuke 4.2 series

PHP-Nuke 6.5 and up