Every writer is unique, with different talents, skills, needs, situation, and so on. Theme, thesis, or thread are totally irrelevant for many writers. They may be utterly essential to others.
Indeed - I've only ever written one book knowing what the theme was beforehand. Mostly I finish the first draft and then re-read it to find out what it was I wanted to write about....then I tighten that theme in the edits. Others can't write without knowing the theme first. Whatever works for you.