I created some new staff banners and put the css in extra.css as Brogans guide showed. The new banner class would not load. I had to move the custom banner css into userbanner.css and once I did that the new banner css would load.
It has the same specificity but as long as it comes after the original definition, it's fine. It it comes before, you can bump up the specificity. (!important being an example of this, though a non-ideal one.)
Otherwise, if that's not something you want to get into, editing the CSS directly is always an option (and perhaps the simplest version).