We've been on other Thomson ships and the dress code is smart for restaurants in the evening. One night is DJ's and long dresses, but only if you want to. The buffet bar is casual dress all day.
Having sailed on this vessel last September. We found smart casualto be the order of the day. It was not necessary to dress up. Having said that there was one Captin's night where it was HOPED that a suit for the men and a suitable long dress fpor the ladies was the norm.
Enjoy the trip.