When someone asks you what the theme of your story is, it can be a hard question to answer. This doesn’t mean your story doesn't have one, it just isn't overt, which isn't necessarily a bad thing. In fact, theme isn’t something you want front and centre.
That is, the reader doesn’t need to know from the outset what themes you’re going to be looking at. And even if by the end they can’t really put their finger on exactly what the overarching theme was, they just have a feeling that they can’t quite put into words, that’s fine. In many ways that sort of response is preferable to being too obvious or predictable.
However, for the writer, it’s important to know how theme is created, how you can shape it and what the mechanics are.