The theme of the play, according to G.R. Elliot is that a "wicked intention must in the end produce wicked action unless it is not merely revoked by the protagonist's better feelings, but entirely eradicated by his inmost will, aided by Divine grace." This is seen most clearly in Act V, Scene 1, where the Doctor says, "More needs she the divine than the physician." It also seen throughout the play in Macbeth's murderous plots.
Also rampant through the play is the idea of "Fair is foul, foul is fair." Basically, this means that appearances can be deceiving. What appears to be good can be bad, and this is seen in such things as the deceptive facade of Lady Macbeth and in the predictions of the witches.
For a sample analysis of one possible Macbeth theme, take a look at this essay