Carnival is a good 30 something line & I would say a good choice. The Americans particularly when in the Caribbean are rarely 'Formal' by British standards & I would be suprised if many Tuxedo's are on display. Carnival have dropped 'Formal' this year & call it 'Elegant' Night where jeans are acceptable, so make of that what you will. As for Christmassy, the ships in Europe do the Tree Bauble & Streamer thing, so I would assume they do over there. Maybe someone has the definitive on that score.
Hi Wilba, don't know what ships you have sailed on in the carib. but I was on RCI Voyager last year and the Americans dressed to kill EVERY night. The men sported tux and the ladies dressed beautifully in evening gowns. My sister who has never cruised before was absolutely thrilled 'cos she had spent a fortune on evening wear and had been a bit bothered that the ship would be a "tad informal" There was a lot of Canadians on board and the 'class'competition between them and the Americans was quite funny .
I would suggest either Carnival or Ocean Village for an informal atmosphere and for younger people. The only difference would be that on Carnival it is mainly Americans onboard and on Ocean Village it is mainly British onboard.