That's not an Add-On, it's done CSS wise.
It basically calls an image from somewhere and puts it behind the user profile.
XenForo Requests
XenForo Avatar Background Picture
Submitted by xTamue, 08-02-2019, 10:36 PM, Thread ID: 120705
Thread Closed
RE: XenForo Avatar Background Picture 08-02-2019, 11:14 PM #2
08-02-2019, 11:14 PM
#2