You don't need a lot of light, but you need some. Part of that is to be able to see what you are doing when you are in there and the door is closed, but a big part is that they need to be able to see to go to bed. They don't start to go to bed until it is ready to get dark outside. If you have a really dark coop, they can wait until it is too dark in the coop to see, so they refuse to go into the coop. A window to let in light is a big help in that.
I like a coop that is not brightly lit. They seem calmer if it is not too bright. I do have a window that I open during the summer for extra ventilation, but windows are not necessary for ventilation. There are plenty of other ways involving vents that you can use for ventilation. But if you do use a window for ventilation, whether a sheet of plexiglass you remove or one that you can open, be sure to cover the opening with hardware cloth to keep predators out.