Dictionary.com defines theme as a subject of discourse, discussion, meditation, or composition; a unifying or dominant idea or motif found in a work of art.
What I find most useful about theme stems from combining two ideas drawn from the work of Lagos Egri and Stanley D. Williams: that a theme emerges only the end of the story and contains a moral premise.
The theme is proven at the end of a story because that’s when the conflict between the protagonist and antagonist is decided. It contains a moral premise because the conflict itself is, at its core, a conflict between good and evil.
In simple terms, if the antagonist wins we have a down ending — evil triumphs. If the protagonist wins we have an up ending — good triumphs over evil.
Establishing the theme in 30 Days of Night
In the film 30 Days of Night the isolated northern Alaskan town of Barrow is beset by a band of vampires intent on using a month of darkness to gorge on the unsuspecting and helpless community.
The sheriff, Eben Oleson, the story’s protagonist, confronts Marlow, the leader of the vampires, in order to protect his town, but clearly lacks the strength to defeat him. All seems lost until Eben hatches a plan to bolster his own strength by infecting himself with tainted blood, turning himself into a vampire. Eben defeats Marlow then purposely exposes himself to sunlight and dies, ensuring that he himself never becomes a threat to the humans.
The theme that emerges at the end of the story is that death, through self-sacrifice, leads to a greater, more transcendent victory by granting life to others.
Isolating themes in this way allows us to see the essence of stories at a glance. It helps us to keep narrative events on track.
The theme embodies the moral premise of the story and is established at the end of the tale.