This is from Carnival's website, FAQ's:
Is there a dress code for the dining room?
Depending on the length of the cruise, either formal dress or casual resort wear is required in the Dining Room. The dress requirement for children is the same. For those not wanting to dress up, the Lido restaurants are open nightly (with the exception of the last evening of the cruise), and have a casual dress code. Formal Wear: tuxedo (optional); suit and tie; sport coat, tie and slacks; evening gown; cocktail dress; pantsuit Casual Resort Wear: sport shirts and slacks; dresses; skirts; pantsuit NOTE: Shorts, t-shirts and bathing attire are not permitted in the Dining Room during dinner.