Usually this is produced by the management. Normally a black T so backstage crew can wear it, it'll have the show's and the theater company's names on it, maybe dates, often an image, sometimes a bit of an in-joke. This can be pretty cool. (Or sometimes really lame.) I have a small collection, including a nice Shakespeare Dallas Twelfth Night T (black) with:
"Some are born great,
Some achieve greatness,
And some have greatness
thrust upon them."
But my new fav. is one designed by one of the actors in the recently closed God of Carnage.
This actor's character took a lot of grief throughout the play's run for fictionally setting his daughter's hamster free into the wild... or abandoning it in a gutter... Pick one. Anyway. This fun T is his rebuttal:
Image by permission of Mark Fickert www.markfickert.com