When considering theme, as an exercise regarding reading or writing, we might be helped by looking at the major themes in some classic works of literature.
The Pride Before the Fall: Various ideas relating to power are present in some of the most significant works of Western literature. The classic works Moby Dick, King Lear, and Heart of Darkness all take up the thematic subject of individual desires for power.
Each of these works explore the issue of an unnatural desire for power, or a desire to escape or overthrow natural human limitations. Similar themes anchor many works of science fiction as well, from Frankenstein to Jurassic Park. This category of theme also includes ideas of overcoming limits.
Identity Issues: Very often in classic literature, themes related to identity formation and the challenges encountered on the way to self-definition/self-discovery function as a focus. Identity is a complex concept and, for the most part, identity formation is a universal human project. Characters from Theseus to Jay Gatsby have been used to explore the various ways that dreams, societal pressure, family and fate can coalesce in an individual in highly interesting ways. Not surprisingly, many young adult novels deal explicitly with identity issues including the Lord of the Rings trilogy and A Separate Peace.
The Inner Life: A third category of theme relates to the mental, emotional and psychological conflicts that can shape characters and plots. In Modernism, this theme often takes an explicitly psychological bent with the stream-of-consciousness narrative style. Arthur Miller's work is largely characterized by an interest in how a personal internal dynamics cause conflicts in the external world.