Background message to a specific user

.message-background (@mbUserName; @mbBackgroundUrl; @mbBackground-size; @mbBackground-position: false) {
 
  .message[data-author="@{mbUserName}"] .message-inner {
    background-image: linear-gradient(to top, @xf-contentBg, fade(@xf-contentBg, 60%)), @mbBackgroundUrl;
  background-position: @mbBackground-position;
  background-size: @mbBackground-size;
}
.message[data-author="@{mbUserName}"] .message-cell.message-cell--user {
   background: transparent;
}
}
// Example
.message-background (admin; url(https://i.imgur.com/W1yqxKS.jpg); cover; left);