I went on the cruise last year and unless they have changed since then, you dont get sent anything saying what theme nights you will have. if you dont get sent anything just call P&O like a week or so before the cruise.
we had a superhero night, hawaiian night and pyjama party. they all were awesome especially the pyjama party but especially the superhero night - mmm hot cruise staff in small costumes....
oh yeah and there were two formal nights. I think the first one was on the first or second night and was a cocktail party a traditional Captains welcome or something and the other was in the middle somewhere for no reason that i can remember