"D minor is the saddest key" - Nigel Tufnel.
That stuff is mostly bullshit - although quite funny in places. (Sorry, Christian!)
The idea that C major is "Completely Pure. Its character is: innocence, simplicity, na´vety" derives from a pianist's view, because it's all the white keys. Therefore it's the most basic, beginner key.
It's nothing to do with the SOUND of the music (except by psychological association).
E.g., as guitarists, all we have to do to transpose from C major to Db major is put a capo on 1st fret. Are we really to believe that suddenly turns the mood of our song into "a leering key, degenerating into grief and rapture. It cannot laugh, but it can smile; it cannot howl, but it can at least grimace its crying"??
(It's OK, one more fret up will give us "the key of triumph, of Hallejuahs, of war-cries, of victory-rejoicing" - whoop-de-doo!)
These ideas did have meaning in the days before equal temperament. (Just intonation, mean-tone temperament, etc.) In those days, half-steps weren't exactly half a whole step, and there might be two sizes of half step; and keys really did sound different from one another. The remote keys (more sharps and flats) became more out of tune, with more complex frequency relationships, and hence "darker" sounds.
By 1806 (AFAIK) equal temperament was well established, and Herr Schubart should have known better (IMO).