My son wore kahki pants and a jacket on formal night and my daughter wore a dress above the knee. I'm sure you could wear whatever you wanted but most people dress up sooooooooo much.
I found that they didn't even dress down after the dinner and all I wanted to do was put on some comfy cloths and go to the shows. I would say every evening even casual night the people on my ship dressed up very nice. So you have to go with the flow...