Menu Underline for current page or current page ancestor?

I’d like to have the same underline that happens in the hover state of the menu be there when you are on current page or current page ancestor (for sub menus).

Wondering what the CSS is to make this happen. A simple bottom border goes too wide. I basically want to hook into the same CSS for the :hover state but make it apply to menus with drop downs if you are in that section and stay on in that section.