Xtreme has an option for background extension for all its main areas (header, footer, menu bar, content..). It is the “Full Width BG” box.
Then as far as creating the same within the content, the theme cannot do anything about it as it does not manage content, but there is an easy way to use custom CSS on content.
What you do is the following
Add the rules below in Main Options > Fonts & Custom > Custom CSS Rule box
.is-desktop #content, .is-desktop #container {overflow:visible;}
.is-desktop .full-width-bg {margin-left:-500%; margin-right:-500%;padding-left:500%;padding-right:500%;}
body.is-desktop {overflow-x:hidden;}
Then when creating content, use the following HTML code for each Container with a different Background color needing to be extended
<div class=”full-width-bg” style=”width:100%;background-color:grey;”>Your content</div>
I added this piece of tutorial to the full width one below
http://forum.weavertheme.com/discussion/10342/weaver-xtreme-tips-and-tutorials-part-1#Item_1
@weaver, given this is a popular demand, may be you could add the full-width-bg class style to the theme