We just returned from a cruise on the Century. After the first informal night, my SO ditched the jacket and tie because he was overdressed in comparison to what the majority of the men were wearing. He ended up donning a dress shirt and slacks for the informal nights and that truly was what most of the passengers were wearing. We had the late seating, so perhaps there's a difference in what's worn at the main seating.
We found that people were not nearly as dressed up as we expected. The formal nights were really the only nights we saw people truly dressed as suggested by the ship. We saw some men in tuxedos, but they tended to be older passengers. For the most part, the men wore dark suits for formal nights.
We learned a great lesson in packing for any future cruises as most of what my SO intended to wear for informal nights returned home unworn.
As for the temperature in the dining room, it was cool and pleasant. I never heard a man complain that it was too cold. Some women, including yours truly, found that a shawl or dressy poncho sufficed and kept us quite comfortable.
