How to Change Gridmarket Heading Fonts

Hi,
I've created my own child theme based on gridmarket and am having trouble changing the heading fonts.
Firebug is telling me that the css 'file' is mydomain.com/store cart#5.
I've tried dropping this into the custom.css of my child theme, but its not making any difference.
h1, h2, h3, h4, h5, h6 {font-family: arial,serif; font-weight: bold;letter-spacing: -1px;}

Any ideas? thanks
Paul