A good way to express themes is by noting how an author uses a literary element to convey an idea. In other words, consider one of the central themes to the novel -- human descent towards savagery. Then, ask yourself how the author used a literary element to convey this theme. A good example for a thesis or an assertion within a thesis would be, "Golding uses man vs. man conflict to convey how humans will inevitably descent towards savagery with the lack of a structured society." Then you would need to provide a specific example of that conflict and analyze/explain (do not summarize) what exactly you mean.
heyy ur answer to the novel is the theme was to trace the problems of society back to sinful nature of man