Preheat the oven to 425°F (220°C).

In a large bowl, combine olive oil, garlic powder, onion powder, smoked paprika, dried oregano, salt, and pepper.

Add the chicken thighs to the bowl and coat them evenly with the spice mixture.

Spread the seasoned chicken on a large sheet pan.

In the same bowl, add the sliced bell peppers and red onion, tossing them in any remaining spice mixture, then spread them on the sheet pan around the chicken.

Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and the vegetables are tender.

While the chicken and vegetables are baking, prepare the herby ranch sauce by mixing together the Greek yogurt, mayonnaise, dill, parsley, lemon juice, salt, and pepper in a bowl.

Once the chicken is done, remove the sheet pan from the oven and let it cool slightly.

Slice the chicken into strips and warm the pitas in the oven for the last 5 minutes if desired.

To assemble, place some chicken and roasted vegetables inside each pita and drizzle with herby ranch sauce before serving.