Well, it’s kind of hard to sort out when you don’t have the new version enabled, but try this in your child theme style sheet or custom css:
#slider_root {border: 1px solid #dddddd;}
a nice simple border with a shadow
On second thought, it looks more like a shadow than a border:
#slider-root {box-shadow: 0 0 3px #dddddd;}
You will probably have to fiddle with the settings on those, or even combine them, to get the effect you want.