the mind > mental capacity > perception or cognition > faculty of ideation > topic, subject-matter > [noun] > leading theme (3)
figurative. The chief theme; leading idea; prevailing sentiment.
The prevailing tone or main idea of a speech, piece of writing, course of action, etc. Cf. key, n.1 18a.
gen. A distinctive, significant, or salient theme or idea; a recurrent or prevalent characteristic.