1. Open a new image, make it transparent and as wide as you expect your web page or blog is. The height can be about 10 pixels as the gradient will flow down the page.

2. Click on Styles > Gradient tool.

3. Edit the gradient as you wish. Use the clock hand.

4. Go to the Flood fill tool and fill the image.

5. Save and use as your background.

Here’s resized example