After you've made the change, you'll see an immediate preview of this change in the preview window.
If you like it, Save your changes. You'll be asked to give your altered theme a name if this is the first time you've changed it.
And there you go. You will now have a different background color.
If you want to get fancier, you can replace the background with an image instead of a color (this is more advanced than replacing with a color). To do so, change the background line so it looks like this:
background: #000000 url( ) no-repeat;
Then upload your background image via the Files column on the left side.
Once the image is uploaded, type the full name of the image within the parentheses. Include the file extension (.jpg, .png, etc), too.