Footer: Add space between footer columns on mobile

Example:

After:

Code:

/* -- code to add additional spacing after footer columns on mobile -- */
@include at-query($max, $small) {
.site-footer .grid__item {margin-bottom:1rem;}	
}
/* - end - */

Where do I add the code?

Use this link to learn where exactly to add this code snippet:

Where to add your CSS style code