signature tune vs theme song

signature tune and theme song both are nouns.

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

As nouns, signature tune and theme song are synonyms defined as:

signature tune and theme song: a melody used to identify a performer or a dance band or radio/tv program

Other synonyms of signature tune include signature.

signature tune (noun)	theme song (noun)
a melody used to identify a performer or a dance band or radio/tv program	a melody that recurs and comes to represent a musical play or movie
	a melody used to identify a performer or a dance band or radio/tv program