All 3 Uses

theme
in
The Mark of Athena
(Auto-generated)

Really, theme parties in general are out.†
p. 125.1
They were alone except for Coach Hedge, who was back on the quarterdeck singing the Pokemon theme song.†
p. 261.4 *
Instead, all the way across the Atlantic, he'd done pretty much nothing except make small talk with sharks and listen to Coach Hedge sing TV theme songs.†
p. 352.9

Definitions:

(1) (theme as in: theme of the novel)
a basic idea that underlies what is being said or done -- especially in a literary or artistic work
(2) (theme as in: musical theme)
the initial or primary melody of a musical composition

or

(as in "theme song") music that is most repeated in a film, television series, or other theatrical production
(3) (theme as in: wrote a 2-page theme on my summer)
an essay written by a student

(This sense of theme is much rarer in recent decades than in the past.)
(4) (meaning too rare to warrant focus)