Darkness oftens serves as a symbolic means to represent evil (black = evil, white = goodness). Of course, darkness can also serve to show a mysterious or eerie air of uncertainty; it can also serve as a shroud or cover for activities that you may want to be kept hidden. In Chapter 15, darkness represents all of these moods and possibilities. The lynch mob obviously chose the cover of darkness to hide, or at least partially disguise, their activities; they could have chosen to approach the jail during the daytime hours if they were not worried about being discovered. The author also uses the darkness to create a bit of mystery about the town, since the children are rarely allowed to visit this part of Maycomb at night.
I suppose the "solitary light" from above and under where Atticus sits serves as a symbolic use of goodness--"solitary" meaning both the light and the single man who attempts to thwart the evil intentions of the gang from Old Sarum.