I don't understand how to make the room darker.

Don't know what instructions you followed but I find the easiest way to do it is in the render interface. In the render interface choose 'night' and then choose a night time environment. Or you can remove certain lights by going to render interface, choose 'add light' remove the lights you don't want--it might be the ones over windows and doors and save the plan to use every time you make a render.