The ubb tags in the faq work by default because they are hard coded. They all don't appear as a option when posting. You either need to know them or look at the faq.
But they can be added.
Either as a hack or I believe in the code tag editor in the new versions.
Control Panel » Content Rebuilder -tab- Code tag editor.
You would need to know regex to enter it which I am no expert by any means.
Gizmo has a code tag import using indent for the tag editor at ubbdev somewhere that could probably be modified to use center.
Or maybe just find the regex in the scripts for center and add that to the editor. Not sure at this point what file hass all the ubb code in it right now but it probably is either in the includes or libs folder.
