History: Additional layout layers feature

            Since Tiki 2 or so, there has been an extra div around module boxes, with the class ''box-shadow''. Now in Tiki 3.0 there are more such divs in tiki.tpl, for adding additional styling, as follows, and a switch to activate them all:

* div id="main-shadow"
Just outside ''div="main"'' (surrounds the whole visible page)

* div id="header-shadow"
Just outside ''div id="header"'' (surrounds the page header and its logo, site title, top menu, etc.)

* div id="middle-shadow"
Just outside ''div id="middle"'' (surrounds the middle of the page containing the left, center and right columns)

* div id="tiki-center-shadow" 
Just outside ''div id="tiki-center"'' (surrounds the center-column content)

* div id="footer-shadow"
Just outside ''div id="footer"'' (surrounds the footer (bottom bar) area

These divs are activated to be styled by checking|ticking the checkbox on the Look and Feel admin page. They are then ready to receive styling from themes that recognize them. 

Activating this option will have a global effect (the site's templates/tiki.tpl file will present the additional divs), but presumably they are ignored if a theme doesn't specify anything for the above selectors.

Ref: http://doc.tiki.org/Shadow+Layers