theme vs idea

theme and idea both are nouns.

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

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

theme: a unifying idea that is a recurrent element in literary or artistic work
idea: the content of cognition; the main thing you are thinking about

Other hypernyms of theme include thought.

As nouns, theme and idea are synonyms defined as:

Other synonyms of theme include melodic theme, musical theme.

theme (noun)	idea (noun)
a unifying idea that is a recurrent element in literary or artistic work	the content of cognition; the main thing you are thinking about
the subject matter of a conversation or discussion	your intention; what you intend to do
an essay (especially one written as an assignment)	an approximate calculation of quantity or degree or worth
(music) melodic subject of a musical composition	a personal view
(linguistics) the form of a word after all affixes are removed	(music) melodic subject of a musical composition

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