One of Shakespeare's themes in A Midsummer Night's Dream is reality vs. illusion. We see this theme presented all throughout the play. One example is seen in the mechanicals who have illusions of presenting a splendid play. Bottom envisions himself being able to move the audience to tears with his performance, as we see in his line, "If I do it, let the audience look to their eyes; I will move storms" (I.ii.22-23). However, the reality is that, due to their lack of education and experience, the mechanicals present a very comical tragedy that is unconvincingly performed. Hence, we can say that the mechanicals' illusions does not match reality.
A second theme we see presented is the foolishness of mankind. We especially see this theme presented in Puck's famous line, "Lord, what fools these mortals be!" (III.ii.116).
Similar to the above is the theme of the irrationality of love. We especially see this theme presented in Demetrius's treatment of Helena. We learn in the very first scene that Demetrius was courting Helena before he began pursuing Hermia and that Helena is recognized as being just as beautiful as Hermia. Hence, Demetrius has absolutely no rational reason for his sudden choice.
A fourth theme we see relates to either the benevolence or the maliciousness of the supernatural. As literary critic Nothrop Frye out, Elizabethans still believed that May Day eve was a night infested with spooks and those spirits could either benefit mankind or harm them. We see the fairies doing both in A Midsummer Night's Dream. We see Oberon wanting to mend Helena's broken heart and bless the lovers, and we see Puck muddling things up.