This is actually built into Xenforo using CSS no plugin needed.
Let's say i wanted to make the admin group a color with background color and a moving small image in it i would do it like this:
In the Xenforo admin panel look in: Groups and Permissions/User Groups
Select the user group to edit and in the Username CSS block you can put:
text-shadow: violet 1px 1px 10px;
color: red;
background: url('
https://i.imgur.com/08Uazn8.gif');
Enjoy hope it helps.