In jail, there is no pure. There is no good, so to say. Light is associated with humanity, peace and happiness. Would one think that jail is meant to be filled with happiness? No. Darkness is an epitome of bad, like a sinner in a church. You wouldn't see a joyful person inside a prison. Instead, it is filled with anger and regret. Miller sets the jail in darkness to create tension, bringing a gloomy mood.
