I do leave the windows open during the day, I don’t close them until night. I have not added a light inside, I’m willing to try it though as well as prop the ramp up.
Your chicks are well old enough to stand a little breeze, so don't close up that side door unless you have no choice (i.e. the opening facing oncoming winds, for example, due to where you placed it). Not sure if that coop has any other ventilation? Actually I'd add more openings if at all possible - the type and placement will depend on your wind direction(s) and general temps/climate. For safety you'd need to reinforce any additional vents with hardware cloth or small opening welded wire, and/or cover them with louvered vent covers screwed in place.