Okay, "background story of one character in a bigger story." That's fine. But what's the character doing? What obstacles will they face, and how will they approach any solutions? Maybe just look at the themes in the link provided and pick one that seems to fit. Remember, there's nothing stopping you from changing it later.

For example, maybe the character is going to get divorced. Then the theme could be, for instance, relationships. Or betrayal. Or friendship, because friends get them through. That sort of thing.