Well, last week when I started writing about finding your story with theme, I failed to define the term. I don't think I made a big deal about defining other terms, such as characters, but theme is iffy. People often have trouble working out just what it is. What it is not is a moral or lesson. Teachers and people of the cloth deal with lessons. Writers do not.
Theme is more of a life issue that the writer is interested in exploring. To go back to last week's examples, an author writing about the impact of divorce on children should be able to do so without hammering readers with a moral lesson on the subject. Working a story around a quite unsavory character who commits one mind bogglingly generous act can raise the question of whether or not people can redeem themselves in such a way without insisting that, yes, anyone can do it.
In The Plot Whisperer, Martha Alderson says that theme reflects the story's view about life and how people behave, which is a far cry from teaching a lesson about life and how people should behave. She also says that a character's transformation over the course of a story often involves theme.
According to Alderson, if you're working on creating a plot (which we haven't discussed yet) before you sit down to write, you can try to work out where theme comes into the story, just as you work out where action comes in. That will be discussed more thoroughly another weekend. But the point I want to make here is that in order to do any planning with theme, you first have to know what theme/world view you want your story to reflect. So it's better to give that some thought before you start writing then to decide after you've finished that, Oh, I guess that's my theme.