Background logo

How do I add a logo to the background of my site, like has (you can see it if you view their site on desktop)

They’ve done it by customizing the CSS:

Admin > Customize > Themes > Edit HTML/CSS (CSS)

and in their case, they have this:

section#main {
    background: url(;
    background-repeat: no-repeat;
    background-position: top right;

html {
      background-color: #f3f3f3;