I've been on quite a number of Celebrity cruises... and honestly have never seen anyone head to the showroom on formal nights in shorts and a t shirt.
Can't imagine that you'd be turned away... as they don't specify that the entire ship is formal dress on formal night. Especially since they offer a casual alternative, and they can't transport you from Lido deck to your cabin <G>
Interestingly I believe Princess does mention in their dailies that suggested dress codes apply to the entire ship after 6 PM... which really makes no sense because, again, they offer the Horizon Court as a casual alternative.
The cruise line's really seem to have no interest in enforcing their suggested rules of behavior anyway... so you should have no problem ignoring which ever ones your choose.
Of course, you won't object when someone sits down next to you in the showroom and lights up a cigarette <VBG>