There may be lamb on both menus but it's not the same dish. I've always enjoyed the lamb rump but others have reported it as fatty. I'd rather have Rack of lamb from the Grills!
Ice White is a formal night but don't let the theme worry you too much. Your husband will be fine in a dark suit and you could wear white/silver/blue. It can just be accessories if you don't want to wear those colours.
Alternatively ignore it, the themes are just suggestions.
If I try to book this cruise I get this option but it doesn't specify which airport. Did you book direct with Cunard or via a Travel Agent ? They might be able to add it to your booking if you haven't already purchased the transfer.