Widen gutters

Great theme! I’d like to widen the gutters between columns to a fixed width of 30px. I noticed the margin-left width on .col is 1.6%. When I change this to 30px, the layout breaks (although I thought employing box-sizing:border-box would prevent this). I’m not sure the exact math involved in the grid system widths.

Any ideas on how to simply widen the gutters?