Added faces for mu4e
Added faces for mu4e

I only need to define 3 faces, the other faces of mu4e look fine
because they inherit from other faces defined in this theme

zzgraph/emacs-theme-gruvbox,greduan/emacs-theme-gruvbox,zzgraph/emacs-theme-gruvbox,zzgraph/emacs-theme-gruvbox,greduan/emacs-theme-gruvbox,greduan/emacs-theme-gruvbox