what worries me in your question isn't the 'thigh', but you said 'fast food'.  What fast food were you planning on eating?  'cuz the only fast food I know of that uses chicken thigh is deep-fried places like KFC, or ground-up and formed like a chicken patty sandwich (also deep fried.)  Both of which are not BFL-approved.

On the occasion I have chicken thighs, it's from a whole chicken I roasted or bought a rotiserrie one from the store.  There is more fat on the dark meat.  White meat is leaner.

No, chicken breast is the leanest part of the chicken, and since lean meats are the only meats approved for BFL, I wouldn't do thighs.

The food list explicitly says chicken breast and the book only mentions white meat chicken, so I consider that the only acceptable part of the chicken to eat.

If you compare the two, one boneless, skinless thigh has 13g protein and 6g fat. A boneless, skinless breast (one half of the entire breast technically) has 27g protein and only 3g fat. BIG difference.

If you are eating fast food, I would suggest looking for other options. Fast food restaurants are also most likely serving dark meat chicken with the skin, which increases the fat to a whopping 10g (or more) per thigh!

If you must do fast food - you can get grilled chicken breasts at KFC.  Also El Pollo usually has breasts.