Backgrounds are very easy to work with within CSS. If you are doing each page individually, all you have to worry about is what you want for each page, but we'll do this tutorial thinking that you've got a separate CSS file and all pages link to it.
So, if you want to change a background color, the code will look something like this:
body {background-color: red}
And if you want to change it to a background image, then the code will probably look something like this:
body
{
background-image:
url('images/background/natfl248.gif)
}
If you want to do more with your backgrounds, check out http://w3schools.com/css/css_background.asp