I would guess you upgraded from GB?
Im not big on themes but going from one system too another that have so many changes would make the old themes not work becuase of all the changes.
Systems might look and feel the same but they are normally different and that causes themes not to work.
A theme is multiple pictures that needs to be correct to the system and answer correctly back to every file its suppost to match up with.
And GB and ICS are two different systems with probably two very different themes so i dont think that would work.
And its a reason why people make ICS themes and GB themes and not just themes.
ICS is very different indeed. I am quite irritated that Whatsapp notifications are totally screwed in ICS. It is a choice of endless notification sounds or total silence. I miss GB when I could set Whatsapp group chats to silence while individual chats have notification sounds.