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Attendance of theme nights is not compulsory and it is perfectly acceptable to wear non theme attire if you chose to do so .
The reality is that very few people do dress up for theme nights in any complicated manner, apart from the defined formal nights when either dj or lounge suits are appropriate. For specific theme nights most just dress smartly but if you are really keen to dress up I would suggest you make do with accessories that you either take with you or buy on the ship eg a mask if it was a carnival theme. Most important thing is to be yourself and to have a good time.