I wouldn't worry so much about it. On Carnival, you will see a wide range of attire on all nights, including formal. I think that most of the folks who are so emphatic about dress codes are not Carnival fans and that is fine. We often enjoy it partly because of the casual attitudes. So wear what you have and dress it up as much as you can with jewelry, sparkly shawl, whatever. But, keep your focus on having fun!