There are 15 meanings listed in OED's entry for the noun theme, three of which are labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.
a subject of a talk, a thought, or a piece of writing; topic: He returned to the theme of American values. a unifying idea, or the most obvious point, as in a work of art, etc.
Definition of theme. English dictionary and integrated thesaurus for learners, writers, teachers, and students with advanced, intermediate, and beginner levels.
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1. a subject of discourse, discussion, meditation, or composition; topic. 2. a unifying or dominant idea, motif, etc., as in a work of art.