Shadows Behind All Content Boxes

I would like to add shadows behind all the content boxes to help the site not have such a flat look to it. I would like this to be consistent for all pages throughout my website… https://classifiedsmaximus.com

Someone was kind enough to offer this code below that I installed in the CSS (under General > Layout Setup > Couston CCC/Java) that worked great for putting a shadow below/under the bottom of the top nav:
#core_header_navigation {
top: 0px;
left: 0px;
width: 100%;
background: #FFF;
z-index: 90;
box-shadow: 0px 0px 15px #333;
position: fixed;
}

How do I do the same for the rest of my site? Thank you in advance! Greg.