Kick and scream all you want, but the Maitre D isn't going to entertain your complaint on the first evening. I have to believe you realize that. We've been on cruises and have seen the MD stop those wearing shorts trying to enter the MDR on the following evenings and turn them around. Kudos to the staff.
We had a lady on the second leg of our 2018 Eclipse cruise advise () my wife and I (while we were waiting to enter the MDR on casual evening) that we didn't have to be dresses formally as it was casual evening. I said, no you don't say!! And I then advised her Celebrity no longer has formal evenings. We walked away shaking our heads. We had our reasons why we elected to dress formally that evening.