What I am referring to when I say “Main Theme areas”, are things like Header, Container (holds content and sidebars), Menus and Footer (I added it to the tutorial for clarity, thank you)
These have their own options pages under Appearance > Weaver Xtreme Admin. In these options you will find the extend BG attribute one.
Of course, since Xtreme implemented all its options in the customizer, in order to see the Xtreme Admin pages, you need to have installed the Weaver Xtreme Theme support plugin.
This plugin holds all the page and post pages options, shortcodes, AND the Legacy Admin pages.