Chemical/liquid rockets and compressed gas thrusters are two different things with different functions. You’re right that lighting a fire in space won’t do much but make a cool firework. A thruster doesn’t so much push against space as it does nudge the ship in the other direction. Newton’s 3rd law and all that except the second “body” in this case isn’t something physical outside the ship but rather the force of the the gas leaving the thruster nozzle causing a recoil.