"A story doesn't need a theme in order to be good" I'm only saying this once but a theme isn't some secret coded message an author weaves into a piece so that your English teacher can talk about Death or Family. A theme is a summary of an idea in the work. If the story is "Susan went grocery shopping and saw a weird bird" then it might have themes like 'birds don't belong in grocery stores' or 'nature is interesting and worth paying attention to' or 'small things can be worth hearing about.' Those could be the themes of the work. It doesn't matter if the author intended them or not, because reading is collaborative and the text gets its meaning from the reader (this is what "death of the author" means).

Every work has themes in it, and not just the ones your teachers made you read in high school. Stories that are bad or clearly not intended to have deep messages still have themes. It is inherent in being a story. All stories have themes, even if those themes are shallow, because stories are sentences connected together for the purpose of expressing ideas, and ideas are all that themes are.