Let Guests Change Theme

[AlexC]

I have it set up so my guests can edit the theme - but the minute they move from the "you have successfully changed the theme" page, it goes back to the default page.

I have been told that the key to making that work is to add another table to my phpadmin thing.

However, nobody has told me what, where or how. Can someone please explain in idiot langauge how to do this? I can use phpadmin, and I know how to add stuff, but I need to be told in babysteps what to add.