There are loads of different themes. Selflessness, Bravery, Honesty, Intelligence and Peacefulness. They all kind of merge into one. It sounds like I'm just listing the traits of each faction, but they point is that these traits can't exist without eachother. People can't be labelled as being Brave, and not have any of the others. Yes, their bravery might be their strongest trait, but that doesn't mean that they exist without selflessness, or honesty, or intelligence or peace. They are brave when they have to be; they don't go around starting fights just because they have Bravery and other people don't. It's not how society works.
I've worded this really badly, but I'm trying to say that the theme is to prove to people that they aren't just ONE thing. They don't have to be selfless and nothing else. They can be selfess, brave, honest, etc, all in one. For example, telling someone the truth: It's selfless because it will have an effect on their opinion of you, but it's honest because you aren't lying and keeping things from them that they might want or need to know.
So yeah. The theme of Divergent is that people aren't split up into different groups, like the city in the books. People have lots of different traits, and some are more promenant than others, but they are all still there.
There are many themes that play in Divergent. The ultimate point of the factions (spoiler alert) is that no one faction can exist without the other, just like no society can exist in this world without the selflessness, fearless, etc.
Overarching themes present:
Society and Class
I think that the main theme is freedom of choice. When you get down to the basics everyone has a choice to make in how they live their lives, and to become the person they want to be. This is shown by the Divergents. But, many people in the Factions decide to make their own choices as well.
The whole them of the story can be said in one word. 'Choices'. the basic slogan of the book is "one choice can transform you" i think it can be agreed upon that the theme has to do with choices. thank you.
I think that sacrifice is a huge theme in this book. Both of Tris's parents sacrificed themselves for their daughter and in the end (spoiler) Tris sacrifices herself for her brother.
Another big theme here is fear. Tobias in particular is always battling with his fears, sometimes in an obsessive way. He constantly goes into his fear landscape to try and conquer these fears but we later learn he never truly gets over them. One of my favorite parts in the series is when Tobias faces his fear of heights for real when ziplining/ scattering Tris's ashes. This kind of shows how much Tris meant to him in a way.