theme (n.)

early 14c., "subject or topic on which a person writes or speaks," from Old French tesme (13c., with silent -s- "indicating vowel length" [OED], Modern French thème) and directly from Latin thema "a subject, thesis," from Greek thema "a proposition, subject, deposit," literally "something set down," from PIE *dhe-mn, suffixed form of root *dhe- "to set, put." Meaning "school essay" is from 1540s. Extension to music first recorded 1670s; theme song first attested 1929. Theme park is from 1960.

Definitions of theme
theme (n.)
the subject matter of a conversation or discussion;
his letters were always on the theme of love
Synonyms: subject / topic
theme (n.)
a unifying idea that is a recurrent element in literary or artistic work;
it was the usual `boy gets girl' theme
Synonyms: motif
theme (n.)
(music) melodic subject of a musical composition;
the theme is announced in the first measures
Synonyms: melodic theme / musical theme / idea
theme (n.)
an essay (especially one written as an assignment);
Synonyms: composition / paper / report
theme (n.)
(linguistics) the form of a word after all affixes are removed;
Synonyms: root / root word / base / stem / radical
theme (v.)
provide with a particular theme or motive;
the restaurant often themes its menus