Go to each usergroup you want the staff banner to appear in. Under userbanner text type in the name of the usergroup (can be anything. doesn't matter but just type in the name of the usergroup to make it easy). Then under "user banner styling" select "other, using custom css class name".
Type in the name of the class you want to use in that box (you don't need a beginning period. Just the name of the class). Then simply reference the class in extra.less like so. You do need the beginning period in the extra.less part. Here's an example of my rank for administrator I use: