Add margin on both sides of the page

Hi, How can I add margin to bothe sides (left, right) of my entire website ? I already have a CSS code to reduce the horizontal space in-between my column blocks, which I'd like to keep, but how can I keep the original width of the pages without affecting this horizontal space ? It looks like those two settings are dependent to each other. Can you help on this ? https://bastienchilloux.com