Though theme seems a simple concept, it can be challenging to learn to isolate and identify a theme in a work. On top of that, many works can have many themes, though generally works have one overriding theme that is more important than other minor themes, not always, but often. Kip Wheeler Ph.D. of Carson-Newman College has a good definition of theme:
A central idea or statement that unifies and controls an entire literary work. The theme can take the form of a brief and meaningful insight or a comprehensive vision of life; it may be a single idea such as "progress" ... [or] a more complicated doctrine, such as "Socialism is the only sane reaction to the labor abuses in Chicago meat-packing plants" (Upton Sinclair's The Jungle).
Wheeler emphasizes the point that the theme of a work communicates the author's intentions and ideas to the reader: it is how the author underscores what he wants you to understand from what you read. So, for example, in the story of "The Three Little Pigs," what the author wants you to understand is that it is important to build homes (and, metaphorically, life) upon sound principles that may take a little more work to implement but that will protect you in the long run. [The theme is as long as the story!] Can this be expressed in a single word or two? Perhaps the idea of "solid foundation" or "permanence" or even "wisdom" might express the idea in a single word or two.
This shows one reason why themes are often difficult to master and identify: there are many words that may describe a theme that is implied. Some themes are expressed directly, however, as in Wheeler's example from Sinclair's book: this is a sentence contained in the text of the book itself. In "Beauty and the Beast," the theme is often described by one or both of the adages "Don't judge a book by its cover" or "Beauty is as beauty does." Both fit the circumstances as the heroine is repulsed by what she sees without knowing what is in the heart, while the hero perhaps didn't behave so well to begin with.