What is the theme of Six of Crows?

Themes in Six of Crows include justice and teamwork.
Justice
Justice is an important theme in the book. Kaz has worked his whole life for revenge⁠—which he sees as just⁠—against the man whose actions led to his brother's death. Nina wants justice against Fjerda for what they do to Grisha. However, their mission doesn't necessarily go as planned. Mathias believes that what the Fjerda do is just; however, he has to learn that just because he grew up with that point of view, doesn't mean it's the right choice. Ultimately, he makes his decision when he faces off with his former commander.
Teamwork
Teamwork is also a major theme. Every member of the heist has to work together to pull off the seemingly impossible plan. When things go wrong, it's only through their personal connections and abilities that they're able to complete the mission and escape Fjerda. However, that teamwork doesn't develop overnight. Kaz, Jesper, and Inej have a built-in trust already because they work in the Dregs together. Mathias and Nina have to overcome their difficult past to learn to trust each other.