forums -> topics -> postContainer:
Change:
<aside class='ipsComment_author cAuthorPane ipsColumn ipsColumn_medium'>
With:
<aside class='ipsComment_author cAuthorPane ipsColumn ipsColumn_medium' style='{{if $comment->author()->member_group_id == 4}}background: linear-gradient(to bottom, rgba(15,183,255,0.2) 0%,rgba(15,183,255,0) 100%);{{endif}}'>
Change background and member group id to whatever needed.