By default, the HTML and CSS structure is setup to support full-width backgrounds per section. The HTML will look similar to this:
<div id="header">
<div class="wrap">
<div class="title">
<!-- your content begins to go here -->
</div><!--end .title-->
</div><!--end #header .wrap-->
</div><!--end #header-->
And the corresponding CSS will look like this:
#header { width: 100%; position: relative; clear: both; }
.wrap { width: 960px; margin: 0 auto; }
Notice how the .wrap class keeps all of the content inside the #header div contained to 960px while still allowing the #header div to span the full width of the page. This type of styling is what makes inLine support full-width backgrounds per section. It is highly recommended that you follow this same HTML structure in order to keep the layout consistent throughout your custom child theme edits.