The process of writing a book (2/6): Theme and concept

Once you start seeing storytelling principles in movies you can’t unseen them—and the same happens with books, TV series, TED talks and any other creative endeavor.
Steven Pressfield in his book Nobody Wants to Read Your Sh*t does a fantastic job explaining how a story is structured. Throughout the book he wants to hammer home the need of figuring out the theme and concept as soon as possible. Because they’re the two main components that tie your story together and drive your writing.
This is worth emphasizing: The theme and concept are the first two things you have to figure out. They will shape your story and guide you through it all the way, by solving a simple (and tough) question: What is it about?
Let’s dissect it in two ways: external and internal.
The concept is the external part. It includes the narrative, characters, style, and all the stuff that shapes a story. As Steven Pressfield says “it frames the material and makes us look at every element of that material from a specific point of view.”
For example in the TV Series Black Mirror, each episode is different (which means it has different concepts in each one.) For example, the concept of the social credit system episode is: “Let’s take a girl who is obsessed with her social ratings, and give her opportunities to hit up her score by masquerading her persona, becoming her worst version.”
On the other hand, the theme is internal. It’s the core of the idea. It takes away the narrative, characters and unnecessary stuff. Continuing with our example, the theme of Black Mirror (on every episode) is: “At the pace technology is growing we’re going to deal with serious social and existential issues, so we better start taking this seriously.”
Figuring out your theme and concept is one of the most difficult parts. It’ll take a while but you can only do it at the early stages—otherwise it’ll be too late.
So, what is it about?