Colored messages for Admins and Mods

On this forum, Admins and mods have a different colored background to their messages - a gradient that starts from the left. How can I add this feature to my own forum?

http://www.malware-site.www/t17-how-to-modify-the-post-background-of-a-specific-user

but the specific tutorial only teaches us to change the color of the background, now the image of the background in which this forum uses.

Okay, I'll read that. I'll try to change the code they show me into using an image, should be something like adding "background-image:" code to it, if I'm right.

Thanks!