From The Art and Popular Culture Encyclopedia
In literary theory, a theme is a broad idea in a story, or a message or lesson conveyed by a work. This message is usually about life, society or human nature. Themes are the fundamental and often universal ideas explored in a literary work. Themes are usually implied rather than explicitly stated. Deep thematic content is not required in literature; however, some readers would say that all stories inherently project some kind of outlook on life that can be taken as a theme, regardless of whether or not this is the intent of the author. Analysis of changes in dynamic characters can provide insight into a particular theme.
A theme is not the same as the subject of a work. For example, the subject of Green Eggs and Ham is "green eggs and ham are well worth eating, no matter the location". The theme might be "have an open mind".
Themes differ from motifs in that themes are ideas conveyed by a text, while motifs are repeated symbols that represent those ideas. Simply having repeated symbolism related to chess, does not make the story's theme the similarity of life to chess. Themes arise from the interplay of the plot, the characters, and the attitude the author takes to them, and the same story can be given very different themes in the hands of different authors. For instance, the source for Shakespeare's Romeo and Juliet, Matteo Bandello's The Tragical History of Romeus and Juliet gave the story the theme of "the wickedness and folly of marrying without parental consent",
And to this end, good Reader, is this tragical matter written, to describe unto thee a couple of unfortunate lovers, thralling themselves to unhonest desire; neglecting the authority and advice of parents and friends; conferring their principal counsels with drunken gossips and superstitious friars (the naturally fit instruments of unchastity); attempting all adventures of peril for th' attaining of their wished lust; using auricular confession the key of whoredom and treason, for furtherance of their purpose; abusing the honourable name of lawful marriage to cloak the shame of stolen contracts; finally by all means of unhonest life hasting to most unhappy death.
but in Shakespeare's hands the same story acquires the theme of "the wickedness of feuds and parental heavy-handedness in preventing young love from marrying."
While thematic analysis is a primary concern of literary critics, a minority viewpoint holds that explicitly stating the theme of a work universalizes it in an inappropriate way. For example, many love stories end happily when the hero and heroine marry, thus the theme "Marriage equals happiness." Critics would point out that marriage rarely does simply equate to happiness and that marriage and happiness are individual and cultural intangibles that may or may not relate.
- The similarities between fantasy and reality worlds
- The loss of innocence that comes with the end of a childhood
- The impossibility of certainty
- The mystery of death 
- (The evils of) Racism and slavery
- Intellectual and moral education
- The hypocrisy of "civilized" society 
- The coexistence of good and evil
- The importance of moral education
- The existence of social inequality
- Alienation as a Form of Self-Protection
- The Painfulness of Growing Up
- The Phoniness of the Adult World 
- Wayne C. Booth, The Rhetoric of Fiction
- Thomas C. Crockett, Walking in America: Themes of Literature