You also have the theme of the loss of innocence. The boys are in their teens, usually the age when childish innocence is gradually replaced by rebelliousness and cynicism. The war years have taken their innocence too soon, leaving them so disillusioned that all they know is destruction. Because destruction is a normal part of their lives, it doesn't affect them. The boys are desensitized to ruin because they've lost their ability to imagine a better world.
Another theme is what happens when power changes hands. Blackie originally controls the gang, but he doesn't encourage the boys to do anything that would hurt anyone. In his hands, power is the ability to lead others. When T. takes over, power changes to the ability to destroy. Power also shifts in the community. Mr. Thomas formerly had power as an elder in the community, but with the war, that power is lost. The boys forcibly take the power from people like Mr. Thomas, creating a chaotic, lawless society.