Many authors don't have a theme in mind when the come to a story. They are often more interested in the genre, setting, action, or characters. But without a theme to glue the pieces together, a story seems to meander aimlessly, not covering specific ground but just heading off in almost any direction.
Your theme has two parts:
1. The topic of the story.
2. The moral or message of the story.
The topic is the broad subject area explored, which may be material or conceptual. Death, the complexity of society, nuclear power or man’s inhumanity to man are all potent thematic topics.
In contrast, the moral or message of a story explores an individual human quality such as greed, self-sacrifice, conceit or compassion and provides the author’s judgment as to whether or not that quality is a good one to possess.
Before you finish lining out your plot, be sure to choose a topical subject matter element that fits in with the material you have already developed and will serve as unifying focus. And also include examples in several of your scenes (spread throughout your story) that illustrate a specific human quality and clearly convey your point of view on its value.