.userBanner { css here}
.XenBase .messageUserBlock .userBanner { css here}
I'll give it a try... used all the guides from xenforo nothing seemed to work.
The other thing you could do is just edit the template: user_banners.css directly, normally i'd recommend try keeping all your CSS changes inside extra.css but something like I'd say go for the template directly. You need to change the background + the little: .userBanner.bannerBlue.wrapped span