The default template html5 is a responsive design and it works well on all screen sizes. You can see for yourself in two ways:
this is a tool for testing how a site looks on differnt devices - try it out on your abantecart or the demo abantecart
http://mattkersley.com/responsive/You can also simply drag your browser to a smaller size window to see how it reacts on small screen.
There is also a mobile template for sale in the extension store - you can get to their thru the Admin Panel.
See if one of those solutions will work for you.