diff-refine-face is not distinct enough

[Dan Nicolaescu]

Please make diff-refine-face more visible, it is very hard to see now.
Given that this face is for showing subtle differences, it would be
better if it would make such differences more visible.  

This face is hard to see on both a white background and on a black
background.

As a suggestion: I tried grey70 and it worked fine on both white and
black backgrounds.  The exact color is not that important as long as it
is not too similar to the default face, or the diff context face (or any
other face in the diff grey world that might appear close to it).