How to make theme wider

Do not edit the theme itself. First create a child theme for your changes. Or install a custom CSS plugin.

I have created a child theme already. Now what?

Leaf theme options > Theme Layout > wide

This eliminates the background on the website

.site {max-width:1100px;max-width:110rem;}
.row {max-width:1100px;max-width:110rem;}

This should be enough, in CSS custom option area.
But I haven’t tested on mobile nor on major browsers, this is up to you.

PS:

This eliminates the background on the website

It’s because that option is the maximum width. Anyway you could change the page background color.