There doesn’t seem to be a border style applied. But there is a box shadow applied.
@media screen and (min-width: 960px){
box-shadow: 0 2px 6px rgba(100, 100, 100, 0.3);
}
You can change the box shadow using the first 3 numbers The first value is horizontal shadow which is set to 0 the next value is Vertical shadow which is set to 2px and the 3rd value is the blur.
You can change the shadow to something like this
@media screen and (min-width: 960px){
box-shadow: 0 1px 2px rgba(100, 100, 100, 0.3);
}
which would make the shadow thinner and less blurred.