Fix a bunch of faces

This PR fixes some misspelled face names, removes definitions and
inheritance of undefined and non-existing faces, and changes the GNUS
faces to the non-deprecated variants.

Particularly, this PR removes the `gnus-group-mail-{4,5,6}` and
`gnus-group-mail-{4,5,6}-empty` faces.  I do not know why these were
included in this theme. Gnus at least does not define these faces.

I am not sure about 78816db.  `message-header-from` seems to be simple
misunderstand. I replaced it with `message-header-other`.

But `hover-highlight` is a mystery to me.  I cannot imaging where this
face should possibly be defined in Emacs, and Google didn't point me to
a 3rd party library that defines this face.  Also, this theme itself
does not actually define `hover-highlight`.  Hence, I replaced
`hover-highlight` with a simple `solarized-hl` background color.