General Nouns referring to ideas On the subject ofThings which is being discussed, considered or studied.
Example:She wrote an article on the subject of class
The theme of
Main subject of talk, book etc
Example:The theme of the poem is emigration.
the issue of
Subject/problem which people are thinking or talking about.
Example:There was a lively debate on the issue of globalization in the seminar yesterday.
@Vocabulix