There is a dress code for the main dinning room. On Formal night you need to be dressed up. A suit for the guys a nice dress for the ladies. I have seen many women in pant suits.
On non formal night it is smart casual, no shorts, nice shirt and pants.
If you do not want to dress up you can eat at the lido buffett. You can wear anything there. Maybe even undressed! You will not feel out of place. However the best food by far is in the Formal Dinning Room.