Figure 5
Go up to your menus and click Modify. From there, you will see Page Properties. You can edit more than just the background in page properties. Click it.
Figure 6 - Click to Enlarge
When you click on Page Properties, you can change a lot of things, but if you just want to change the background, then look at where the red arrows are pointing. You can just change the color (by typing the word or the code [#000000, etc] ). If you want to use an image as your background, make sure that image file is already on the server before you link to it.
Now, start a new page with a XHTML 1.01 DTD, and follow the steps on this page. Make the background whatever color you want. Then, go to the next part.