the best way to do this is to code your own menu and add an extra <div>. If the site will be static and there won't be many new pages, this will work.
Instead of adding the pages widget, add a text widget with the following
HTML Code:
<div id="pages-2" class="widget widget_pages">
<div id="my-inner-box">
<div class="widget-title"><h3> </h3></div>
<ul>
<li class="page_item page-item-2 current_page_item"><a href="http://www.bigmamas.com.au" title="Welcome">Welcome</a></li>
<li class="page_item page-item-6"><a href="http://www.bigmamas.com.au/food" title="Food">Food</a></li>
<li class="page_item page-item-8"><a href="http://www.bigmamas.com.au/menuwinelist" title="Menu/Wine List">Menu/Wine List</a></li>
<li class="page_item page-item-10"><a href="http://www.bigmamas.com.au/functions" title="Functions">Functions</a></li>
<li class="page_item page-item-12"><a href="http://www.bigmamas.com.au/reviews" title="Reviews">Reviews</a></li>
<li class="page_item page-item-14"><a href="http://www.bigmamas.com.au/contact" title="Contact">Contact</a></li>
</ul>
</div>
</div>
then change the CSS Insert to this
HTML Code:
div#pages-2 {
border:solid 1px #000000;
}
div#my-inner-box {
background:red;
margin:20px;
}
the outer <div> will have the border and the inner onw will have a margin and a red background. You can fiddle with the magrin size to get it the size you want.