Sorry. I am totally new to CS-Cart.
I've inherited a site and its templates are located in a folder:
/design/themes/responsive/blocks/ etc
Is it correct to edit the *.tpl files in this location? Is this a custom theme area which is not overriden by an upgrade?
Your supposed to use the theme builder that is in the admin section of Cs-Cart. Here is some documentation that explains the process a little further. http://docs.cs-cart.com/4.2.x/themes/index.html