How to add space between the portfolio featured images in Zelle

If you want to add space between portfolio featured images then, add the following CSS to Appearance -> Customize -> Additional CSS Section.

.cbp-rfgrid li {
    width: -webkit-calc(98% / 4);
    width: calc(98% / 4);
    margin-right: .5%;
    margin-bottom:.5%;
}

@media screen and (max-width: 1190px){
.cbp-rfgrid li {
 width: -webkit-calc(100% / 4);
 width: calc(98% / 4);
  }
}
@media screen and (max-width: 1024px){
.cbp-rfgrid li {
  width: -webkit-calc(98.5% / 3);
  width: calc(98.5% / 3);
}
}
@media screen and (max-width: 768px){
.cbp-rfgrid li {
 width: -webkit-calc(99% / 2);
 width: calc(99% / 2);
}
}

Result: