SiteOrigin CSS + UNDERSCORES BASED THEME

Hi!

I use visual editor. I tried to make a design, but I did not succeed.
In visual editor no fields margin: 0 auto; max-width: 1200px;

Please write a video as a basic theme to make a good design.
https://underscores.me/

Thanks!

.site {
    margin: 0 auto;
    max-width: 1200px;
    background: #f0f0f0;
}