The themes of The Fall of the House of Usher are the typical representative themes of Romanticism and 19th century Gothic literature. They include:
1. The inevitability of death
2. The authority of fate vs. our own free will
3. Nostalgia and sadness for better times gone by.
4. Inevitability of disgrace (as if destiny had precluded the characters to settle for it)
5. Darkness as a foreshadowing of the end, and as an atmospheric stylistic device.
** All these are staple themes of Romantic Gothic Literature which permeate through all of Poe's major works just like the works of his peers at the time.
In my opinion, there are a couple of major themes in this story. I think that the story is about insanity and evil. These themes combine to make it a story that is also about terror.
These two themes are shown in the characters of Roderick and Madeline. They both seem to be at least somewhat insane and we see Roderick getting less sane as the story goes on.
But why are they like this? That is where evil comes in. Poe seems that there is something that is simply evil or cursed in the Usher family. It is not clear why this is -- some say it is supernatural, some say it is because the family is incestuous and inbred. But somehow, there is an aura of evil in the house.