What do you mean that they get "so much treats"? Chickens' main diet should consist of chicken feed, either layer or starter or, in many cases a good all around food like Flock Raiser. If she's getting too many non-nutritious treats, then she will be thin. Leghorns are thin birds to begin with and fairly high energy so she may never get "fat" but will stay rangy.
If you think she has worms you can worm her, but you could be sure by taking a fecal sample to a vet. Most vets, even if they don't treat birds, can check a fecal sample for parasites. I don't like to worm unless it's necessary.
Also check to make sure her poop is okay because that's a good indication about nutrition. There's a poop chart somewhere on the forum. She may need more grit if she's not out running around. I give mine access to grit even if they run around the yard.
My suggestion is to cut back on the treats - way back. A little scratch at night before bed to fill up the crops, some scrambled eggs or cooked grains once and awhile. Feed a good quality chicken food, either a layer mix or Flock Raiser and make sure that that's the majority of their intake and make sure they have access to plenty of fresh water.
Good luck,
Mary