If I have a glittery gif how can I put it on the background of my username?
You'll need to make the .gif glittery image yourself and then just add the image in the front of "Username Style". But tbh, it's not a good thing to add. Sometimes browsers can't comprehend them properly (When I had it, Opera viewed all the usernames as sparkly image names and not just mine). My advice is to not add it.
Yeah you're probably right.