Settlers will start with a rudimentary shack, dark and dingy, but eventually (if your settlement is safe and has enough power, water, food, and defenses) add their own lights to the interior. Plots for homes, stores, and industrial buildings come with an electrical pole attached, so you just hook up one power line to each, connected to a generator, and you're set as far as electricity goes.
Your settlers feel more like real people and less like nameless, corn-shucking automations. This is why the mod should become an official part of the game: it makes your settlements feel alive, far more than the original game does. Sort of like, y'know, a real settlement would. I think that's my favorite thing about the mod-seeing what the settlers build for themselves, and how it changes over time. Looking for more ways to customize, tweak, improve, and enhance your game? Check out our list of the best mods for Fallout 4.