musical theme vs melodic line

musical theme and melodic line both are nouns.

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

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

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

musical theme (noun)	melodic line (noun)
(music) melodic subject of a musical composition	a succession of notes forming a distinctive sequence