Edit the template "navbar" and find the snippet:
Code:
<div class="body_wrapper">
'
You'll see it just as you posted then paste your div w/ css id or class included above it.
To get there go to:
AdminCP > Styles & Templates > Style Manager > *Drop down to "Edit Templates" in the drop-down menu beside the style name, now look for the "navbar" template it's in the template family "Navigation / Breadcrumb Templates ?"
That's the template you need to edit

.