A work of literature may have more than one theme. Hamlet, for instance, deals with the themes of death, revenge, and action, to name a few. King Lear's themes include justice, reconciliation, madness, and betrayal.
While you might start with an issue or theme in mind, themes will also develop or emerge as you write. It may not be until the editing stage that you even begin to recognize your themes. Having recognized them, your themes will help you determine what to cut from your story or novel and what to highlight.