A theme is an idea or perspective developed by a piece of literature. A theme is not the same as a topic. Rather, it is a perspective on a topic. For instance, "love" is not a theme by itself. It requires a perspective on love. "Love is blind" is a perspective on love that could be a theme. So is "the futility of love" because it expresses the worldview that attempts at love are always doomed. "Fear" is another topic that could be the root of a story's theme, but to express fear as a theme, the writer analyzing the text should take time explaining what the story seems to reveal about fear. In Star Wars, "fear leading to the dark side" is a theme that Darth Vader's story arc expresses. Vader feara losing Padme when he is Anakin Skywalker, Jedi Knight, and because he cannot conquer his fear, he makes choices that lead him down path of figurative and literal darkness. This theme could help viewers to understand that fear, if not conquered, can lead to some destructive choices. Hence some themes can be read as lessons in wisdom. However, it is important to remember that the aim of literary studies is not to brainwash scholars into a way of thinking but rather to give them opportunity to explore the complex, diverse world of perspectives.
As long as the theme a student finds in a piece of literature can be well justified with textual evidence, (is shown to be present in the text), it is most likely an acceptable theme. However, while identifying theme, it is important to remember that the perspective or worldview represented by the text may not always belong to the author. Some authors prefer to take a walk in other people's shoes while they write, which may account for a theme being a reflection of a perspective not their own. Others may have intended to push one worldview, but the components of the text may have come (by unconscious means on the author's part) to represent a different perspective. Know if your teacher expects historical context to be taken into account before you write about a theme. Some literary scholars believe a text is always alive and breathing and thus able to gain new themes as contexts around the texts change to inform the interpretation. Other scholars do not agree that theme can be separated from historic context, occasion of publication, or authorial intent. Both are acceptable views with merit. It is always helpful to know about perspectives that dominated the literary era in which the work was written, the historic moment, and the author. However, it is not always necessary, as the text can be read with a purely Formalist approach.
To identify a theme, first read through the text and note ideas that seem to reemerge. For a story, thematic ideas may emerge in the plot, character, setting, symbolism, and figurative devices. For a poem, theme can be found in literal and connotative content, figurative devices, and form or structure.
A theme statement will not only name the perspective on the topic but will also name the components of the text that revealed the pattern. For instance, "Jealousy is a green eyed monster" is a theme in Shakespeare's Othello that could be identified in plot action, characterization of the protagonist, and figurative language. To analyze its presence in plot, a writer would want to relate how Othello's murder of Desdemona is a monstrous act of jealousy. He thinks she is unfaithful to him. Next, the writer would want to take time explaining how Othello's character becomes increasingly jealous as the play progresses. While he was once confident in his love and his spouse, he grows to be insecure and suspicious by act 4. These are two characteristics of jealousy that can warp a normal person or make them seem monstrous. Finally, the writer would want to address the metaphor within the theme statement itself, as it is derived from a line in the play.