Add full color configuration for XnView

[revell]

Of all possible colors computers are almost always grey. And so are most programs. XnView already offers to change various background colors which is great. And I'd find it even greater if really everything could be configured including the explorer background color. I don't know if that is possible but at least the menue bars.
I'd like to set up a dark black or almost black color for XnView. Images look much nicer then.

[XnTriq]

Related request: Surfaces Change Color