My CSS is still pretty basic, but I think this should work:
.group-Owner img.avatar {
box-shadow: 0 0 0 1px rgba(255,255,255,0.3), 0 0 0 1.5px #F99A07, 0 0 10px 4px rgba(244,148,14,0.75);
}
Though the ‘Owner’ group would have to be set to be a user’s primary group.
(small edit to tweak it for both topics and topic lists 
)