If so, it is :
<style type="text/css">
body{
background:url(Put your background image link address here);
}
</style>
and somewhere inside the <head> </head> tags in your HTML page
There are ofcourse a few more options to the background properties, all depending on what you want to display and how, but in general, what given above suits most needs.
If you just want a color :
<style type="text/css">
body{
background:#00FF00);
}
</style>
Just add in the correct hexvalue of the color you want