Tonight I'm wearing a green hoodie, NATCA convention T shirt, jeans and a pair of green Tiger Onitsuka shoes, exactly like I went to work in at 11am. The contractual language is "Appropriate for the conduct of government business." Whatever that means in a career where you're pretty much sequestered from public view. Others choose to dress more business casual. Some take it a step further and wear the clothes they cut the grass in. I can't imagine a professional pilot walking in, noticing I'm wearing jeans and his only takeaway is "Next time I hear that guy, I'm doing the opposite of what he says!"
Tshirts, jeans, shorts, flip flops, sweats, none of it is prohibited. I work with someone who when in the TRACON will wear a snuggie 6 months out of the year when vectoring. If I, someone you'll never meet, wore a suit you never saw, for a public that doesn't know what a TRACON is, I'm not sure what the advantage is. Kinda like a freight dog hauling boxes not wearing a complete airline pilot uni. I'm pretty sure the boxes don't care if the pilot is wearing shorts. As for me, he can wear ass less chaps and if he sounds like he has a grasp of what he's doing, great!
Edit to add two things.
We wore business casual during the white book imposed contract from '06 to '09. Hopefully I'll never do that again. At my facility we all wore orange on Wednesdays, referred to the TRACON as Home Depot and said things like "Man, the plumbing isle was busy today." in protest.
The other thing is supervisors wear ties. Phrases such as so and so put on a tie have distinct meaning. I've worked with supes who were phenomenal human beings as well as stellar controllers, but by and large a line controller with pride in their work doesn't want to be one or look like one.