Custom Site Margin

Site URL: https://lavender-reed-n64j.squarespace.com/

Hi,

In Squarespace settings there is an option in Design>Spacing to change Site Margin. How may I override this setting in code?

I am looking to make the margins on the left and right larger (20vw?), while keeping the top and bottom small (like 3vw).

Right now I did this: 

#page {
    margin-left: 200px;
    margin-right: 200px
}

But when I zoom out in browser (to test on bigger screens) the margins change size with my code. Without code, the margins stay consistent. I want to keep the consistent behavior, just increase the margins on left/right. 

Site password: 2020

Thanks!