keynote vs leitmotif

keynote

noun

  • The note on which a musical key is based; the tonic. 

  • The main theme of a speech, a written work, or a conference. 

  • A speech that sets the main theme of a conference or other gathering; a keynote speech or keynote address. 

verb

  • To deliver a speech that sets the main theme of a conference or other gathering. 

leitmotif

noun

  • A melodic theme associated with a particular character, place, thing or idea in an opera. 

  • A recurring theme. 

How often have the words keynote and leitmotif occurred in a corpus of books? (source: Google Ngram Viewer )