A theme is a sentence, not a word: a coherent sentence that expresses a story’s irreducible meaning.
Screenwriting guru* Robert McKee prefers the phrase ‘Controlling Idea’, because ‘theme’ is now used widely in colloquial language to mean other things.
WAR is not a theme. War is a setting.
LOVE is not a theme. Love is a genre (Romance, love story)
TEEN DRUG ABUSE is not a theme. Teen drug abuse is subject matter.
The Controlling Idea = Value changed by Cause
The theme exists to tell the emotional lesson of a story.
Value means the primary value in its positive or negative charge that comes into the world or life of your character as a result of the final action of the story.
Cause refers to the primary reason that the life or world of the protagonist has tuned to its positive or negatvie value.
e.g. Justice (VALUE) triumphs (the change) because the hero is smarter than the villain (CAUSE).
The theme of The Artifacts: Hope (VALUE) is restored (CHANGE) because a boy realises the value of knowledge and abstract joys over the amassing of material wealth (CAUSE).
The theme of Midnight Feast: Adult-like awareness of poverty (VALUE) is gained (CHANGE) when a girl stays up late one night and sees the poverty right outside her home (CAUSE).
The theme of Hilda Bewildered: A young princess learns to deal with performance anxiety (CHANGE) when she learns the power of visualisation (VALUE) on the night of her first speech (CAUSE).
The theme of Diary of a Goth Girl: It is only after the grim reaper comes for a pessimistic try-hard goth (CAUSE) that she learns (CHANGE) the value of human kindness (VALUE).
Theme might also be expressed like this, embracing the didactic part of the story:
The Artifacts: It’s better to collect knowledge and experiences than material wealth.
Midnight Feast: It’s fairly easy to ignore poverty even when it’s right outside your own window.
Hilda Bewildered: Difficult real life situations become surmountable once harnessing the power of visualisation.
Children’s literature seems to have a higher tolerance for didacticism (though the trend is against it), so you’ll often find themes written like that somewhere in the advertising copy.
*John Truby thinks in terms of ‘moral argument’ and ‘symbol web’. According to Truby the theme exists to show “The writer’s view of the proper way to act in the world.”