bug#50141: Current part of emacs red, other parts blue

[Eli Zaretskii]

> Cc: 50141@debbugs.gnu.org, jidanni@jidanni.org
> From: martin rudalics <rudalics@gmx.at>
> Date: Sat, 21 Aug 2021 15:55:57 +0200
> 
> (set-window-face-remapping-alist nil '((default (:background "yellow") 
> default)))
> 
> It was pretty trivial to implement but lifting it from the rest of my
> changes might be non-trivial.  If people are very, very interested, I
> can give it a try.

But the request here is not to do this for a particular window, the
request is to do this for _any_ window when it is selected.