This is a semi-transparent PNG for wallpaper; set the background color(s) and whether you want either solid or gradient according to your personal preferences. The image will appear monochromatic/semi-alpha when viewed in a viewer. There is a version available as a jpeg with preset color if you would prefer:
I think it would be neat if Gnome could do a radial gradient or arbitrarily angled linear gradients for the background, maybe multi-point custom gradients as well.
The file is a rather large download; sorry, can't do anything about that.
Comments are appreciated.