Act 2, scene 3 in Shakespeare's Macbeth includes the famous porter's scene as well as Macduff's discovery of Duncan's murder. This particular scene is quite rich in thematic significance. Enotes has a thorough discussion of the prevalent themes in the play as a whole. I have posted the link below.
But for your specific act, one theme that you might examine is that of appearance versus reality. It seems that the guards murdered Duncan, that they were commissioned to do such an act by Malcolm and Donalbain, and that Macbeth murdered the guards out of his grief for Duncan. Of course, we know that these appearances are not true. This theme is relevant throughout the play as individuals such as Duncan, Malcolm, and even Macbeth struggle to determine the difference between appearance and reality in their judgments of others.
Another theme that you might consider is the idea of heaven and hell. What is particularly intriguing about Macbeth is his belief in the afterlife, and his knowledge that killing Duncan does damn his soul. The porter's speech echoes Macbeth's belief in that the porter in his inebriated state feels as if he is in hell.
Lastly, you might examine the idea of gender roles. Early in the play, we see Lady Macbeth assuming a more masculine role as the dominant one in the relationship. However, in this scene, we see the two reverting to their more traditional gender roles: Lady Macbeth faints--which could be an act--while Macbeth brutally slays the guards.