musical theme vs idea

musical theme and idea both are nouns.

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

As nouns, musical theme and idea are synonyms defined as:

musical theme and idea: (music) melodic subject of a musical composition

Other synonyms of musical theme include melodic theme, theme.

musical theme (noun)	idea (noun)
(music) melodic subject of a musical composition	the content of cognition; the main thing you are thinking about
	your intention; what you intend to do
	an approximate calculation of quantity or degree or worth
	a personal view
	(music) melodic subject of a musical composition