How do I add padding around images on gallery page on Flatiron?

Try this code to Website > Website Tools > Custom CSS

#container-content {
   padding-left: 8px!important;
   padding-right: 8px!important;
}
#grid .item {
 margin-left:-8px;
 a {
   border: 16px solid #fff;
   }
 /* Overlay size on hover */
 .wrapper {
   margin: 16px;
 }
}