This is a most interesting topic. We all know that old-time children's books were condemned for being "too preachy." That's one extreme of theme. Works like the old Dick and Jane reading books ("Go, Jane, go," and "Dick runs fast," etc., etc.) have no themes because they are not designed to teach anything except reading.
To me, it feels awkward trying to establish a "theme" in the beginning, yet a writer should be able to state the subject of the work. It may be "jealousy," or "confusion," or "grief," but is should be about something, and the writer should have something enlightening to say about the subject. If they have not, there's no point in writing.
Rather than "theme," I encourage my clients/students to tell me what they are trying to say with the story, novel, whatever. The reader should come away from the work convinced of something other than "I have just read a book." In my feeble opinion, at least.
Forgive me for running on, but I know a most interesting book about an "agony aunt" who is crippled and in a wheelchair. She hires a secretary, who at length falls for her husband, and he for her. In the end, the woman rolls her wheelchair off a cliff and dies. The book seems to have no theme, as such, but it certainly had something to say - or ask - about human motivation: did she sacrifice herself so her husband could be happy, or was it a done to prove a point? The story allows for both possibilities. It's an old book, but the question haunts me to this day.