Preheat the oven to 425°F (220°C). Line a large sheet pan with parchment paper or lightly grease it.
In a small bowl, combine chili powder, ground cumin, smoked paprika, garlic powder, onion powder, dried oregano, salt, and black pepper to make the fajita seasoning.
Place the chicken thighs in a large bowl. Add 2 tablespoons of olive oil and half of the fajita seasoning. Toss to coat the chicken evenly.
In a separate large bowl, combine the sliced red, yellow, and green bell peppers and sliced onion. Add the remaining 1 tablespoon of olive oil and the rest of the fajita seasoning. Toss to coat the vegetables evenly.
Arrange the seasoned chicken thighs on one side of the prepared sheet pan. Spread the seasoned bell peppers and onions in an even layer on the other side of the sheet pan.
Roast in the preheated oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the vegetables are tender and slightly charred at the edges.
Remove the sheet pan from the oven. Let the chicken rest for 5 minutes, then slice the chicken thighs into strips.
Serve the sliced chicken and roasted vegetables with warm flour tortillas and lime wedges for squeezing over the fajitas.