Distinguishing buffer background from text

[Christoph Groth <christoph@grothesque.org>]

Hello,

I'm looking for a way to set the background of a buffer (i.e. all 
that is neither text nor whitespace) to some slightly different 
color. There doesn't seem to be an easy way to do this, as the 
“default” face seems to be used for both.

Motivation: I started to like editing text with visual-line-mode + 
adaptive-wrap-prefix-mode + variable-pitch-mode. This works almost 
perfectly, I’m only missing a way to make superfluous whitespace 
at a _soft_ wrap apparent.

I.e. something      like this, where the soft line break happens 
to occur before “like”.

One solution is to use whitespace-mode’s “spaces” or “space-mark” 
to make all whitespace stick out, but I find this disturbing.

Thanks,
Christoph