What is the overall theme of "The Fall of the House of Usher"?

One very strong overall theme is that of tradition.  The "Usher" family is a symbol in itself.  Over and over in the story, the narrator mentions how long Ushers have lived in the house, aptly named "The House OF Usher"--not of Smith, Jones, etc.  The narrator also mentions multiple times that Roderick and his sister are the last of the Ushers.  Once they die out, it stands to reason that the House can no longer exist.  They are connected...remember how Roderick and Madeline are feeling ill? As their mental and physical health declines, the house itself "feels" ill and displays it in the hairline cracks of the foundation, the overgrown weeds, the darkness that has overtaken it.  The Ushers and the house are one being...and have been for many years...tradition.  Once the Ushers die, the house also crumbles and is swallowed by the ground on which it has stood for years and years.