I like gruvbox, because it improves the readability a lot!
Though the bare scheme does not match all my expectations, therefore I
added some more settings in the muttrc. The scheme itself came from the
official neomutt page.
Furthermore this change fixes the display of the date and the overall
preview to a easier to read format.
Reference: https://www.sthu.org/code/codesnippets/mutt-gruvbox.html
