theme has 14 distinct synonyms spread across 5 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(music) melodic subject of a musical composition
“the theme is announced in the first measures”
an essay (especially one written as an assignment)
“he got an A on his composition”
provide with a particular theme or motive
“the restaurant often themes its menus”
Synonyms, antonyms and the broader and narrower relations for theme come from Princeton University’s WordNet. Full sources and licences.