To replace the background with an image, I personally created a new template and included it in the PAGE_CONTAINER template just after:
<body{xen:if {$bodyClasses}, ' class="{$bodyClasses}"'}>
<xen:hook name="body">
Where I then placed:
<xen:include template="BACKGROUND_VIDEO" />
And with...
After leaving a review stating that the theme seemed to have broken with the newest Xenforo, I reinstalled the theme and customized again slowly and carefully and my issue was resolved. Wonderful theme that I enjoy very much.