Edge backgrounds

Ok, the other day I came across a design that I liked, but figured, why bookmark it, i'll just check my history to bring it back up later tonight... Well I should have bookmarked it....

Here is the gist of it...

It had centered content, to fit 800x600, which I have seen many times, however on the left and right side fo the content, it has a gradient background on each side, going from black on the edges of the browser window, down to a solid color at the edge of the content, and no matter what you resize it to, the solid color is always at the edge and visible, it is the darker edge that gets moved out of the viewing area.

Now I can get it to work right on the right side, but on the left side, when you make the window more narrow, the dark black edge is what remains (on the left edge of the screen), not the solid color on the right of the background, which is up against the content.

Here is an example of how It should be, this first image is how the site looks at a wide resolution:

Now here is the desired look when the window is resized to be narrower:

Sigh, but here is what I get:

Any help on how to achieve this would be appreciated.

-Greg