theme vs subject matter

theme and subject matter both are nouns.

Word	Noun	Adjective	Verb	Adverb
theme	Yes	No	Yes	No
subject matter	Yes	No	No	No

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

Other hypernyms of theme include content, message, substance.

theme (noun)	subject matter (noun)
a unifying idea that is a recurrent element in literary or artistic work	content communicating a message; what something is about
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)	subject matter (verb)
provide with a particular theme or motive