How long have you had the dog? No, idiopathic seizure disorder is not caused by food.
Environmental allergies are not caused by food.
The dog may need an anticonvulsant for the rest of her life. Sure there are triggers, in fact I would avoid vaccines with this dog and ask your vet to sign a waiver (rabies vaccine).
Regarding allergies, see a veterinary dermatologist. Food allergies are rare.
Use the search engine here at this site to look up “environmental allergies” and “seizures” and see my posts.
Unfortunately, these issues may be why the dog was given up. There are good vets out there that will help you.
Bottom line, it will cost about $1000 to $2000 a year to keep this dog comfortable.
I could be way off, as only a veterinarian that has examined the dog and reviewed it’s history can advise you accordingly.
PS: Be careful, don’t fall down the homeopathic rabbit hole.
