It's a custom MyBB code, basic css and html will sort you out
If you mean the "Administrator message by *", you could quote them and possibly remove the swear words and tell them to not to swear. I find it more sensible that only moderators use such "messages".
EDIT: Ignore my post if you'd like this to your website.
You can set them up in MyBB Administrator Panel, then find 'BB Codes' somewhere. I'm used to IPBoard now, haven't used MyBB in quite a while. The only downside to using BB codes is that any user can use them. Somebody has created a plugin for MyBB whereby you can select 'Moderate' as if you were going to edit a post:
http://mods.mybb.com/view/modnotice-1-3