border of the same width all around blocks EVEN BETWEEN COLUMNS

Site URL: https://impala-amphibian-6n45.squarespace.com/work

Hello guys,

I want borders (white in the picture) all around each content block.

My problem is with the width of these borders. I want the width to be the same around the whole pack of blocks AND in between the columns (where the width is currently doubled because borders touch).

How would you do this so it looks like that on desktop ? On mobile it should be a simple proper border since all the blocks are going to stack above on other.

I'm looking for css code to tweak a section theme.

PASSWORD FOR SITE : visitor

Thanks