My son and I just returned from a 3 night trip on the Wonder. The answers I have to your questions may not be entirely correct as I kept things rather casual for my 10 yo DS.
Is there a formal night? Not that I was aware of. Just the dining rotation. Ours was: Tritons, Animators Palate, Parrots Cay.
Is the dinner dressing more casual than on the Magic? Don't know, have never been on the Magic.
Is there a tropical or other theme night? Not that I was aware of in the dining rooms. There were themes in the lounges, such as a '70s night, Latin Dance party, etc.
Hope these answers help some.