The main theme is true righteousness. The religious leaders had this external righteousness based on law. But the righteousness Jesus described is a true and vital righteousness that begins internally, in the heart. The Pharisees were concerned about the minute details of conduct, but they neglected the major matter of character. Conduct flows out of character. Jesus gave this message to believers.
Instead of beginning with a negative criticism of the religious leaders, he began with positive emphasis on righteous character and the blessings that it brings to the life of the believer.
He’s basically describing a Kingdom Character.