No changes and no theme change, just open the dialog and ...this is the thing I don't remember, I'm not sure if I tried to just open the dialog and hit Cancel or open the dialog browse 1-2-3 themes and hit Cancel but I think I tried the first option too just to see, ALT+k>Cancel and the nick colors change on the new lines of chat.

All these tests are done on a ZNC also but I don't see why it would be any different with a normal session.