How to make twenty eleven theme with cart responsive

Hello, I’m coming in a project after it is nearly finished. A company I’m now working for hired some developers to do an e-commerce site and they did it based on the twenty eleven theme and installed a shopping cart that isn’t one of the industry standard.

I tested the site yesterday to see if it was responsive and it isn’t. I’m not a coder but I can read instructions and get it done. My question is: I’ve found online tutorials that explain how to make that theme responsive in the CSS. I pointed that out to the developers because if that is all that is required then their quote for doing it is crazy. I got the “that isn’t all that is needed” response I expected but is that all that is needed? I mean it is how it displays the items not the items themselves. Or am I wrong?