You're right- it may be a little uncommon, but it's grammatically correct. I think generally it would be used if it was a specific article of clothing, like a Best Buy shirt "Their blue shirt." It's saying the design or look of the shirt, not a physical singular shirt, belongs to whatever "It" is.
Undershirt is posted in the menu to choose from, why is it considered incorrect? I always knew that chemise is undershirt when I was young. Why include undershirt in the menu and you are disqualifying its usage? I'll appreciate an honest answer. That's why we are here to learn and be confused. Thank you
an undershirt is "une petite chemise/une chemisette" (children's underwear), "un maillot de corps" (for men, with or without sleeves), "un débardeur" (no sleeves) "un T-shirt".
the prime meaning of "shirt" is "une chemise" = men's top with buttons on the front, long buttoned sleeves and a collar, to be worn primarily with a suit.
women's shirt = un chemisier