[TWood]

Usually the background is set to a color. Then create a 1 pixel wide by however tall you want the gradient portion to be, image. The gradient in the image should start at some color and then end at the same color as the background so it blends in. Then make it the background-image, align top left and repeat-x. That means it will start at the top left and then repeat horizontally (x direction) across the page. Here at vBulletin.org, it looks like they have gradients set to repeat-y (vertically) down each side of the body.

EDIT:
BTW, Adobe Fireworks is a great program for doing this. There are probably other programs too, but they should work in hexadecimal (#121212) web colors, and not just RGB (000 111 222) colors to get everything using the same system.