T-shirts and shorts are specifically mentioned as not being acceptable for any dinner seating. Blue jeans are generally allowable on the casual nights. On all casual nights, a collared shirt of some type and long pants are the expectation.
For formal nights you will at least need slacks, shirt, tie, and a sport coat. Two formal nights on a 7-day cruise. One formal night on cruises of lesser duration. But fear not, because if you want to stick to a t-shirt and dungarees you can always go to the buffet for dinner. They serve an evening meal in the buffet which is really pretty decent, and normally has many of the items that are on the main dining room menu.
The beauty of Carnival is in the dining room,You don't need a tie or jacket.I have been on 10 Carnival ships and my husband wears slacks,and a nice shirt only.IT is fine.If you choose to go to formal night ,then you need a suit,or slacks,tie,and jacket.Be comfortable cowboy.Jeans and shorts are good for dinner by the buffet,not dining room.