Calibre questions are best left to those who use it. It has been a long time since I used it for conversions. But yes, I think you would add it in the Extra CSS field. I use Sigil so I can examine the entire stylesheet which is usually a separate file named SOMETHING.CSS. Some stylesheets are easier to modify than others. It all depends on the complexity of the stylesheet. If the publisher used a very complex stylesheet, then placing this statement in calibre's Extra CSS field might not work.