If you want to change Banners CSS then the TuT works. you could also add your own Banner Template to add your changes their, but thats also more complicated...
At the end we cant know what you are doing since you dont tell....?!
for the surrounding effect get rid of all "span" classes like
.userBanner.wrapped span.after etc
for the width change the margins in .messageUserBlock .userBanner
Rest is up to you.
To know what classes i have to work with i use Firebug with Firefox, but Chrome has it too.
Then search for the class inside Xenforo ACP and you will find what you are looking for.
For the Changes use EXTRA.css
Play around a bit, if you mess something you can revert the Templates back to default.
I also would suggest to create an own style or maybe a TestForum before changing anything on a live forum