by loren_d_c » Mon Mar 30, 2015 1:18 pm
Demo stores have the same themes as live stores, so all you would need to do is select a responsive theme. In the Theme picker you can click the thumbnail to see a larger example of any theme, and the ones that are responsive will have a 'Responsive' banner across the corner of the example. Most of the themes listed towards the top will be responsive.
-Loren