We have a fairly minimal dress code here at my library. Jeans are fine, just not ratty or anything like that, and no t-shirts with “non-library related” slogans or symbols on them. This last is open to some interpretation, so the guys running our D&D and Pokemon events have absolutely worn gaming shirts without a problem, but our de facto Teen Programming librarian has gotten a little push back from administration about wearing a shirt with a rainbow pride patch. (I don’t think it’s actual discrimination in this case, but I do think the complainer was trying to ride that ‘moderate’ position of not offending the public. 
)
Of course when the “t-shirt question” has come up in the past, no one has been able to adequately explain the difference between a plain t-shirt and a fitted, solid color, short-sleeve top without buttons.
