theme vs keynote

theme and keynote both are nouns.

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

As nouns, keynote is a hyponym of theme; that is, keynote is a word with a more specific, narrower meaning than theme:

Other hyponyms of theme include bone of contention, precedent, head, question.

theme (noun)	keynote (noun)
a unifying idea that is a recurrent element in literary or artistic work	a fundamental or central idea
the subject matter of a conversation or discussion	(music) the first note of a diatonic scale
an essay (especially one written as an assignment)	the principal theme in a speech or literary work
(music) melodic subject of a musical composition
(linguistics) the form of a word after all affixes are removed

theme (verb)	keynote (verb)
provide with a particular theme or motive	set the keynote of
	give the keynote address to (an audience)