At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga.
Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus.
Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat.
The central theme of this (amazing) poem is extreme terror and its effects on the human mind.
There are other themes too, like Isolation, (being cut off, being isolated from the world of the living, is in part the cause of the narrator's abnormal fear of being buried alive. The thought of being alone and abandoned, without hope of ever seeing another human being, petrifies the narrator) and Letting His Sleeping Mind Lie (At the end of the story, the narrator says the deep recesses of the human mind, where reside unknown and inscrutable terrors, are better left alone, unexplored. Here, Poe seems to be saying that it is unhealthful to dwell on morbidity and vague fears. If one focuses incessantly on an imagined illness, he will become ill. If he focuses incessantly on what could go wrong, nothing will go right. The mind has its own secret hell; to awaken its demons is to awaken bale and bane).
Overall, the main theme of this poem is fear, which is even all self-inflicted, as his terror is the result of his dwelling obsessively on morbid thoughts.