The 'responsive' theme is a base theme that is to be used as a started theme for other responsive themes. It is meant to be used from a 'mobile first' design approach, but adheres closely to the 3 column standard Moodle layout.
Like the 'base' theme, it will not show up in the theme selector. In fact, it aims to support the a large majority of the same styles defined in the 'base' theme, but there are obvious differences.
When creating your theme it is advisable that you start designing for a narrow mobile size first; then design for larger displays (980px is a good mark) and then for the designs in between.
I hope you find this useful and can help the movement towards more responsive layouts for Moodle courses.