In a nutshell, themes are the 'point' of everything you do. For example, when you pick jobs, there's probably an underlying reason that goes much deeper than a paycheque. When you choose a mate or friendships, chances are there's a theme there too. With my writing, there are underlying themes that create a central idea behind by books. Themes are essential because they are the lifeblood of the story. I think the best way to demonstrate this is to give some examples.
In my first two books, Fire and A Spark before the Fire, my theme was how our culture often doesn't see entertainers as real people but rather machines whose every move is meant to be entertainment, their lives on constant display. This happened when I watched a celebrity have a very public meltdown, and people were essentially laughing at her rather than having any concern for her mental state. It frustrated me that we have tabloids and silly shows that focus on everything the stars are doing, and actually, this has only grown worse since writing these two books over ten years ago. Of course, this wouldn't exist if there wasn't a demand.
My current (Hernandez) series eased the reader into the whole world of organized crime and an underground world that people may assume is rare if they even believe it exists at all. The central theme is corruption, crime, collusion, and the belief that sometimes it's hard to distinguish the 'good' people from the 'bad'. Although my books follow a group of criminals, there is a point where readers see that the lines are often blurred. My goal was to make readers think and perhaps ask some questions, if only to themselves. Who runs the world? Politicians, corporations, religious leaders....or is it you? Who controls what we see and what we know? Is it manipulated? These are the questions that I've explored as the series unfolded a few years ago, and I continue to ask myself.
A theme gives us a larger focus, which is essential to both the writer and the reader.