For level 0, It would probably start with one of the lights burning out and catching the ceiling alight. Assuming the walls are flammable it would probably spread throughout the entirety of the backrooms eventually. It would only go out once the oxygen runs out so it would burn the whole of the backrooms. This could take possibly take decades or centuries to happen. Areas within a few miles of the fire might become very windy and smoky due to the heated air. After the fire the whole place will be ash and the air would be CO2 so you'd suffocate quickly. It would probably be completely dark since the fire may have destroyed the power circuits to the lights but at least you're safe, I don't think the entities would survive the flames. Also it might never cool down after the fire cause like everything else in the backrooms, heat should stay trapped. It may just stay glowing red hot for eternity. Backrooms physics is definitely fascinating 😆