Plot can be summarised as the protagonist having a goal, the antagonist getting in the way of that goal, and secondary characters and/or external events helping or hindering one or the other.
Characters generally evolve as I write to have goals, values, histories and quirks, all of which shape their contribution to the plot.
In a way I suppose I could try to figure out the theme based on a simple question: what we learn from the events of the plot, and the characters' reactions to them? Although in that case, most of what I've written would have the theme that it doesn't matter where you've come from or what you've done, it's always possible to move on and make something (better) of your future. Except when the theme is that friendship is what matters over and above anything else.
Maybe one of both of those themes is my 'brand': that essential quality that makes a story a Stevie Carroll story. Writing across genres (or sub-genres, as most of my plots revolve around a mystery of one sort or another), I can't hope to develop a brand as a writer of (mostly) genre X.
Other aspects of the brand: female characters with a strong sense of self, and self-determination, male characters with a healthy respect for women (and who may have been raised in a predominantly female environment), characters who choose what they believe to be morally right over what's easy or what conforms to the exact letter of the law.
Many years ago, I was accused of writing 'car porn': stories in which the make and model of a car was an important plot and/or characterisation point. I'm not denying it, or that I often write characters who do care about the make, model and possibly even year and colour of the car they drive. Not all of them, of course, some might care more about whether their guitar is a Telecaster or a Stratocaster, or whether they live in a house built this century or three centuries ago.
Then there are the locations. Some places are almost a character in themselves, shaping the experiences of the more character-like characters, working with or against the protagonist on their journey, being more than just a backdrop against which the action unfolds.
How about you? What are your preferred themes? What is your brand?