Adding Background Image via CSS

I know how to add a background image to site via the “Customize” option within the theme options, but where exactly in the css code does it get added by doing so? What div id does it attach to? I would like to add this image manually to my custom css, not through the theme options.

Please don’t refer me to a plugin or widget, I need a css answer for this. And yes, I’m using a child theme 😉