Preheat to 425°F (220°C). Line a sheet pan with parchment paper or foil for easy cleanup.
In a small bowl, whisk together olive oil, balsamic vinegar, brown sugar, garlic, Italian seasoning, salt, and pepper.
Add chicken thighs to a bowl or zip-top bag. Pour marinade over and toss to coat. If you have time let sit for 15–30 minutes (or up to 4 hours in the fridge for more flavor).
Arrange chicken on the prepared sheet pan in a single layer. Discard excess marinade.
Bake for 25–30 minutes, flipping halfway through, until chicken reaches an internal temperature of 165°F and is caramelized on the edges.
Spoon any pan juices over the chicken and garnish with fresh herbs if desired.