I should write for an ideal audience whose tastes closely match my own, to mine own self be true. The right choice for a critic is a much thornier problem, since it's far less clear that each critic should declare as good only those books which exactly match that critic's taste. Yet an honest reviewer can, after all, only report the reading experience they actually had. In my extreme youth, I had trouble getting my mind around a definition of theme since I, and the people who were trying to explain it to me, tended to conflate it with plot summary. It wasn't until I became a writer that it came clear.
Plot is what a book's about.
Theme is what a book's really about. And some of my books, by God, had identifiable themes, at least in retrospect. Barrayar, for example, might be described in terms that emphasized its political plot, which involves a planetary civil war, or its societal plot, which involves an off-worlder woman slowly penetrating the meanings of the new culture into which she has been plunged. But the book's theme is about the cost to one's identity of becoming a parent, explored through six parallel couples and coming to a thematic climax simultaneously with the political plot. The Imperial Residence burned not only as the climax to a fight, but as a symbol of the phoenix-pyre by which the main character's old identity a single person was finally annihilated and her new one as a mother forged.