Special face for background of active window

[Deniz Dogan]

An Emacs user in #emacs asked about a feature just now that I myself
asked about a while back, namely whether there is a face for the
background of the currently active window. Apparently there is no such
thing, but why? Are there any technical difficulties in implementing
such a face? I'm thinking since there is `mode-line' and
`mode-line-inactive', it can't be too hard, can it?

The motivation is simply that having a different background face for
the currently active window makes it much easier to see which window
is active on a big monitor.

-- 
Deniz Dogan