Child theme – edit CSS file directly, not in GUI

Is it possible to edit the CSS for a child theme without using the GUI? I'd like to edit the .css file directly, but cannot find it for the child theme I created. I created a style.css file in my child theme, but it's not being applied/used. ps- It's the file used for the "Additional CSS" section that I'd like to edit directly. Thanks in advance. URL www.grayphoto.us