I'm currently trying to do that. The only thing I've found is to do the changes directly into the different templates. So, what you can do is put the whole "body" thing into a table (3x1) and put your border as background in the first and last cell.
in the navbar you have the start:
Code:
<div class="body_wrapper">
in the footer you have the end:
Code:
</div> <!-- closing div for body_wrapper -->
If there is another way, I'm interested as well.