theme vs melodic line

theme and melodic line both are nouns.

Word	Noun	Adjective	Verb	Adverb
theme	Yes	No	Yes	No
melodic line	Yes	No	No	No

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

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

theme (noun)	melodic line (noun)
a unifying idea that is a recurrent element in literary or artistic work	a succession of notes forming a distinctive sequence
the subject matter of a conversation or discussion
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)	melodic line (verb)
provide with a particular theme or motive