"Whats dress code, or more to the point what is the most comfortable/ least formal that is accepted?"
THAT was the question. Not "What should people wear to the theatre?" There is no "Dress Code." You will not be turned away for inappropriate attire. Hence, my answer, "You can wear whatever you want." Absolutely true.
I won't get involved in a stupid "People should dress up" discussion. You know, some idiots think you should dress up to fly on a plane.