On our first cruise we found people generally stuck with the dress theme for the evening (throughout the evening). As the years (only 5) have passed by I would observe both a "lessening" of the interpretation of what is formal, informal and casual and an increase in the "run back to your room and change" crowd.
I find this less true on Celebrity than on other lines, but still find it to be true. Perhaps that accounts for the success of Norwegian's Freestyle, all casual, approach these days.I know my wife likes to "dress" in the evening and I do not mind a bit
matching the formality of her outfits with my own (and staying that way throughout the evening).
Where else but on a cruise is a kid from the south Bronx,going to
get to put on a tuxedo or dinner jacket and accompanied by a beautiful women in an elegant gown, go out for a predinner cocktail, followed by a five course dinner , followed by a show,
and a quiet moment on the stern of a ship under the stars.
Just not the same in a tee shirt and cutoffs.