To create a repeating background for your website using a gradient affect that looks something like this:

do the following.

Adobe illustrator CS5

Draw a square.

Open the gradient window (Window -> Gradient)

Click the gradient box (will apply black to white gradient).

Open the color pallette (Window -> Color) and get both the color and gradient window on screen at the same time (tricky because Adobe UI is terrible).

Once you’ve got them both up, click and drag the colors you want for your gradient from the color palette down to the ends of the gradient.

Transform and slim the gradient until your happy. Save this image somewhere you can access from your website. Remember to reduce the size of your canvas (Shift -O) to make sure your image is slim when you export it.

CSS

Now apply the following CSS to the body of your website:

body {
		background-image: url('../images/bg_body.png');
		background-repeat: repeat-x;
		background-color: #c9efed;
		height: 100%;
}

This takes the gradient we just created, and repeats it across the screen and applies a default color to anything below your gradient image where is runs out.

Voila! That’s it. You’ve now got a nice background to build off of.