My Husband and I were on QV just before Christmas and all the Formal nights (and I prefer that term) were well adhered to - thank goodness. In fact, we saw staff politely asking passengers on a few occasions who were in the Chart Room Bar to leave after 6pm as they were not dressed appropriately. No fuss was made, and the passengers got up and left.
Yes, you can dress in whatever clothes you are comfortable in, but you should then keep to the areas designated for more informal wear on Formal nights and adhere to the dress code for certain areas after 6pm.
I do agree that the 'themes' can be taken for 'fancy dress'. Perhaps it would be an idea for people to use the internet to look up what 1920's styles, for example, actually mean. Items do not have to cost a lot. I purchased accessories from Amazon at extremely reasonable prices, and 1920's style dresses do not have to be 'expensive' (although that term means different things to different people).
Cunard are, unfortunately, one of the only cruise lines that continue to have such a formal attitude, and it is what most of us travel on Cunard for.
As most cruise lines do not now have this attitude, perhaps those people who prefer a more relaxed way of dressing for the whole of their cruise should stick to those lines and leave Cunard to those of us who love dressing more formally.