Changing fontsize on sidebar with Gridmarket

Hi all,

I was just curious if there is a way to change the fontsize of the sidebar (more specifically, the sidebar for product catagories) with the Gridmarket theme.

I'm usually just goto the stylesheet but because this is a child theme it isn't available there. Is it just a case of opening the stylesheet on the parent theme and altering that? Or is there something else that needs to be done before hand?

Thanks in advance!