If you mean the white background (body) and you want the gradient to run from dark at the top to white at the bottom then set the background colour of the body to white in your css. Make a graphic that spans the width of the page and and a height of your choice with a gradient from black at the top to white at the bottom and set that in your css in the body and set to no-repeat. This will blend into the white at the bottom.
Your css could be:
This is just a suggestion, there are many other ways but this is a quick and easy method for you.