I know there is an add-on but my theme is custom made so it doesn't do anything for me. Is there a code in Extra.CSS that would work? I would like to have it hidden for both guests and members combined.
You can modify the add-on to your hearts content as long as you don't distribute it typically. Most add-on authors (and in fact even Xenforo) do not mind extending the add-ons for personal use.
Just realize that any update that comes out for that add-on would have to be extended again - unless you extended the add-on with another add-on.