Hi,
1. Well, HikaShop uses by default the Boostrap library popup system that is included since Joomla 3.0.0. As we were told at that time, the templates were supposed to implement bootstrap by default and thus the popups would work fine. But some templates, like yours, didn't, and thus you get such issue unless to revert to the old popup system of Joomla, Mootools.
2. As far as I know, it has nothing to do with the template or CSS we're using on our demo website. You should get the same thing with protostar, the default template of Joomla. So it might be an issue on your template CSS or something.