You must always know exactly what your work is about. If anyone asks, you must be able to express your theme in one sentence, like this:
‘This [novel, story, poem] is about the futility of life in a post-postmodern world of transvaluated values, and the radical failure of the spirit in the face of human cruelty and cosmic despair.’
If this exact sentence does not describe your work, you are writing the wrong story. Get it right, or throw it out.
H. Smiggy McStudge