Colored scrolled_window

[Eric THIERRY <eric thierry finconsult fr>]

Hi there,

I create a scrolled window and a tree that I add in with a viewport. I'd
like the "background" (what I see on my screen) of the viewport or
scrolled window to be colored, but it desperately stays white.
I tried with creating a style with gtk_style_copy, setting all GdkColor
(i.e to red) and then using gtk_set_style to set this style either to
the scrolled_window and the viewport. I can see a thin colored line
around the big white rectangle in which is my tree, as if this rectangle
was drawn on something that I colored well. What is this rectangle ? Can
I color it ?
I tried many other ways but none succeed.

Could someone help me about it ?

Thanks a lot.

--
Salutations,

Eric THIERRY