I am not certain which part of the theme is unresponsive as you mentioned. Perhaps you haven't started building the pages yet since X and Cornerstone are both responsive out of the box. You might want to try starting with the demo content if you are unsure where to start.
Also, Cornerstone is a great editor which helps user build pages with ease. You may check this video tutorial to get an intro on how to use Cornerstone.
If you are still not able to find X and Cornerstone helpful, you can simply uninstall them:
Hope these helps.