if you open your website in a text editor you can change the background color by changing the number value here: <bgcolor>

you can add this into the background carrots <> to add an image: background="wwwyoursitecom/yourpicture.jpg"

there are a few more tweaks that will allow you to create variations of the above (such as tiling a background image or making it static).

just ask google or use your browser to view source code from sites which will give you a good idea of how other people make their backgrounds