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

[Gregory Heytings]

> I don't know -- I haven't actually looked at how this could be 
> implemented.

It is already implemented AFAICS:

(require 'auto-dim-other-buffers)
(set-face-attribute 'default nil :background "red")
(set-face-attribute 'auto-dim-other-buffers-face nil :background "blue")
(auto-dim-other-buffers-mode 1)

It uses the :filtered feature introduced in Emacs 27.