theme vs melody

theme and melody both are nouns.

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

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

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

theme (noun)	melody (noun)
a unifying idea that is a recurrent element in literary or artistic work	the perception of pleasant arrangements of musical notes
the subject matter of a conversation or discussion	a succession of notes forming a distinctive sequence
an essay (especially one written as an assignment)
(music) melodic subject of a musical composition
(linguistics) the form of a word after all affixes are removed

theme (verb)	melody (verb)
provide with a particular theme or motive