To add a background color that covers the entire web browser’s viewport,  set a background  property to the HTML body tag:
body {
background-color: #333333 ;
}
That create dark grey background color (#333333).
Applying a background color to the body tag  add more contrast between  your web page content and the remainder of the screen particularly with fixed width CSS layouts.  In general, using a CSS background color is the easiest way to “colorize” a given block of content without adding much to your web page download time.

You can also add a background image to your web page body using the CSS style.
body {
background-image: url(images/bgbody.gif) ;
}
The url parameter defines the relative path to the graphic used as a background image.
That file’s path can be absolute (i.e. http://livetrainingsession.com/images/bgbody.gif  or /images/bgbody.gif) or relative to the location the CSS style as show above.
Feel free to copy and paste that absolute path nto your web browser’s address bar to see the actual background image.

In this instance the web page uses a very small graphic made of dark-grey patterns. You can see that tiling bleeding through the left and right edges of this web page if your display width is greater than 980 pixels.

You can  simultaneously apply the background color and the background image to the body tag as well. You would write the CSS style as follow:
body {
background-color: #333333 ;
background-image: url(images/bgbody.gif) ;
}

or use a more compact,  shorthand version:
body {
background: #333333  url(images/bgbody.gif) ;
}

or even more compact:
body {
background: #333  url(images/bgbody.gif) ;
}

In this example, the background color is written in its compact form (#333). An hexadecimal value  made of three pairs of identical values (33) can be replaced by just three values (333), one for each pair. Hence “#999999″ could be replace by “#999″ and the color “#FFFFFF” can be written “#FFF”.

It’s always a good idea to use a background color in conjunction with a background image. The web browser covers the “viewport” with the background color while downloading the background graphic. It’s not only for aesthetic but to avoid the visitor having to stare an empty white screen while the web page is loading.

It’s worth mentioning the background image is always displayed on top of the background color whenever the two are applied to the same HTML element. And it does not matter in which order they are defined.