Sailed with Celebrity last year. Dress code was as follows:-
Formal: Dinner Jacket/Tuxedo
Informal: Lounge suit collar and tie
Casual: Open neck shirts and trousers/no shorts and tee shirts.
No jeans in the main dining room at dinner.
I have sailed with Celebrity 4 times and have never seen anyone in jeans in the formal dinning rooms. But of a day time it wouldn't be a problem. Most people were wearing trousers and polo shirts or shirts in the evening. Women were dressed smartly of a evening. If you wanted to wear jeans of an evening you could always go to the buffet dinning instead which is also great food, just without the waiter service. Formal night is very formal, Dinner suits and evening dresses.