you will need to find this in postcontainer
<article {{if $comment->author()->hasHighlightedReplies()}}data-memberGroup="{$comment->author()->member_group_id}" {{endif}} id='elComment_{$comment->$idField}' class='cPost ipsBox {{if $otherClasses}}{$otherClasses}{{endif}} ipsComment {{if ( settings.reputation_enabled and settings.reputation_highlight and $comment->reactionCount() >= settings.reputation_highlight ) OR $comment->isFeatured()}}ipsComment_popular{{endif}} ipsComment_parent ipsClearfix ipsClear ipsColumns ipsColumns_noSpacing ipsColumns_collapsePhone {{if $comment->author()->hasHighlightedReplies()}}ipsComment_highlighted{{endif}} {{if $comment->isIgnored()}}ipsHide{{endif}} {{if $comment->hidden() OR $item->hidden() === -2}}ipsModerated{{endif}}'>
replace it with this
<article {{if $comment->author()->hasHighlightedReplies()}}data-memberGroup="{$comment->author()->member_group_id}" {{endif}} id='elComment_{$comment->$idField}' class='pec cPost ipsBox {{if $otherClasses}}{$otherClasses}{{endif}} ipsComment {{if ( settings.reputation_enabled and settings.reputation_highlight and $comment->reactionCount() >= settings.reputation_highlight ) OR $comment->isFeatured()}}ipsComment_popular{{endif}} ipsComment_parent ipsClearfix ipsClear ipsColumns ipsColumns_noSpacing ipsColumns_collapsePhone {{if $comment->author()->hasHighlightedReplies()}}ipsComment_highlighted{{endif}}{{if $comment->isIgnored()}}ipsHide{{endif}} {{if $comment->hidden() OR $item->hidden() === -2}}ipsModerated{{endif}}' data-group_id='{$comment->author()->member_group_id}'>
than find this same place
<aside class='ipsComment_author cAuthorPane ipsColumn ipsColumn_medium ipsResponsive_hidePhone'>
replace with
<aside class='pec ipsComment_author cAuthorPane ipsColumn ipsColumn_medium ipsResponsive_hidePhone' data-group_id='{$comment->author()->member_group_id}'>
add to css
.pec[data-group_id="4"] {
background: #b85a5b;
background: -moz-linear-gradient(top, #ab0000 0%, #000000 92%);
background: -webkit-linear-gradient(top, #191919 0%,#000000 92%);
background: linear-gradient(to bottom, #c75050 0%,#a66769 92%);
}
change the colors to what ya want
ID 4 is group id