Thumbs margin

Because the 3D nature of the grid (and because it is impossible to add real margins), this solution is not perfect, but it works somehow. Use this custom css code inside the dedicated area for Custom CSS Code under the Theme Options Panel:

.folio-cube {
   border: 2px solid #fff;
}
#portfolio {
   margin: 1px 0 0 1px;
}

The code above adds a border to the 3D cubes in the grid and also adds some margin to the grid holder, because otherwise it will not be centered. I wouldn't suggest going with a larger "margin" than this, because it won't look good.

Additionally, if you choose a different color for the border (let's say red), you should also change the page's background to that color, otherwise you'll see white spaces when the thumbs rotate.

.page-template-template-portfolio-php {    background: red; }