Update colorscheme of mutt to my beloved gruvbox

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

.config/mutt/muttrc

@@ -10,6 +10,14 @@ set confirm_append = no
 set editor = vim
 set sort_aux = last-date-received
 set mailcap_path = "$XDG_CONFIG_HOME/mutt/mailcap"
+set date_format = "%d.%m. %I:%M"
+set index_format = "%3C %Z %d  %-18.18F  %s  (%-2.4c)"
+source "$XDG_CONFIG_HOME/mutt/gruvbox.muttrc"
+color index_author color167 color234
+color index_size color142 color234
+color index_date color214 color234
+color status color220 color235
+#color markers     color234 color234
 
 auto_view text/plain text/html text/calendar application/ics
 alternative_order text/plain text/html text/calendar application/ics