I use Fusion 9. You need to Open Styles and look (and click on) the Page Backgound. This will tell you in the properties box what the background image is. probably background.gif. Now look in the folder for the website you are working on and in an images folder for that style you will find that background image. Copy it somewhere else and open it in an image editor to alter it. Rename it and save it back to the original folder. You will need to remove the Read Only attribute in the original style and then alter the background to the new one you`ve modified.