musical theme vs melody

musical theme and melody both are nouns.

Word	Noun	Adjective	Verb	Adverb
musical theme	Yes	No	No	No
melody	Yes	No	No	No

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

Other hypernyms of musical theme include air, line, melodic line, melodic phrase, strain, tune.

musical theme (noun)	melody (noun)
(music) melodic subject of a musical composition	the perception of pleasant arrangements of musical notes
	a succession of notes forming a distinctive sequence