An interesting question. Shakespeare uses contrast in characters and atmosphere in many ways. He uses contrast between how characters are in one location (the town) with how they are in another (the forest) to advance the plot. He contrasts how they act in love versus how they act when not in love (Titania, Bottom). He contrasts how they act when under magical spells with how they are normally (the young lovers), and how they act when themselves versus when playing a part (the rude mechanicals in the play). What comes through is that there is often an entertaining shift in action, but an underlying continuity of character. Often the contrast is superficial, as when Titania and Oberon clash; they are as willful and imperious when clashing as when not.
The difference in atmosphere is more subtle, and is driven by these coarser changes in character, and is communicated by changes in language. It emphasizes plot shifts, and establishes meaning.