Dark, smart (not old and battered) trainers will be ok. The minimum for formal nights is collared shirt, tie and jacket. Most will bs wearing a suit and tie, some tuxedos and there is usually a few dress kilts if you look for them. The only places that adhere to the dress code is the main dining room, the Crow's Nest (or equivalent Metropolitan?) and maybe one other bar.
If you don't want to dress up you can use the buffet or, one of the other restaurants that charge a supplement.