Theme develops details because a theme always needs details so when you create a theme it develops details throughout the story.
In the first two chapters, the events that occur (i.e. the boys making fun of Piggy, Jack saying that the next time they find an animal he won't be afraid to kill it, an early power struggle between Jack and Ralph, and the death of a young boy due to Jack's uncontrollable fire) demonstrate the themes of man vs. society, man vs. nature, and the loss of innocence.
