Advice wanted - NO weather in December ?

I think the Dome is kept at 68. I always found it comfortable, but my wife and my sister-in-law are always cold.

Weather in New Orleans in the winter is a matter of cold fronts. If you are here just after a front, it can be a bit cold, but it will gradually warm up over a few days until a new front comes through. Assuming you are now living where there is cold weather, you probably would only need a light jacket or a sweater. You could bring both to be safe.

Most music clubs in New Orleans have no dress code other than you have to wear a shirt. Tipitinas, House of Blues, Jimmys, etc. are all very casual. Some of the hotel clubs might have some code, but I'm not sure. The only place I've seen a dress code in New Orleans are the upscale strip clubs. But, even those only require a collared shirt and no jeans or shorts.

Edit: I just noticed you are from Denmark. You would probably laugh at what we call "winter." You could probably walk around in shorts and a short sleve shirt and be fine.