All literary works have themes or central messages that authors are trying to convey. Sometimes, theme is described as the main idea of a work, but more accurately, themes are any ideas that appear repeatedly throughout a text. That means, that most works have multiple themes.

All literature has themes because a major purpose of literature is to share, explore and advocate for ideas. Even the shortest poems have themes. Check out this two line poem, My Life Has Been the Poem I Would Have Writ by Henry David Thoreau.

My life has been the poem I would have writ
But I could not both live and utter it.

When looking for a theme, ask yourself what an author is trying to teach us or show us through their writing. In this case, Thoreau is saying we have to live in the moment, and living is what provides the material for writing.