theme song vs tune

theme song and tune both are nouns.

Word	Noun	Adjective	Verb	Adverb
theme song	Yes	No	No	No
tune	Yes	No	Yes	No

As nouns, tune is a hypernym of theme song; that is, tune is a word with a broader meaning than theme song:

theme song: a melody that recurs and comes to represent a musical play or movie
tune: a succession of notes forming a distinctive sequence

Other hypernyms of theme song include air, line, melodic line, melodic phrase, melody, strain.

As nouns, tune is a hypernym of theme song; that is, tune is a word with a broader meaning than theme song:

theme song: a melody used to identify a performer or a dance band or radio/tv program
tune: a succession of notes forming a distinctive sequence

Other hypernyms of theme song include air, line, melodic line, melodic phrase, melody, strain.

theme song (noun)	tune (noun)
a melody that recurs and comes to represent a musical play or movie	the adjustment of a radio receiver or other circuit to a required frequency
a melody used to identify a performer or a dance band or radio/tv program	the property of producing accurately a note of a given pitch
	a succession of notes forming a distinctive sequence

theme song (verb)	tune (verb)
	adjust the pitches of (musical instruments)
	adjust for (better) functioning