Hello,

the background is set via css. Unfortunately, there are no option to change this in the admin panel.

Unfortunately, not all options can be created with theme options.

You can try to use this method for your needs:

You should work with the CSS and write them in admin: Appearance -> Customize -> Additional CSS

and browser console (f12) in your theme.

https://atendesigngroup.com/blog/how-determine-which-css-styling-element

Then just write the detected css in Customize-> Additional css or by using child theme. 

So, you will not lose it after the new package of our theme uploading.