Responsive has many of the elements that you know and love from other popular frameworks but the key difference is that they are given only the most basic styling. This means that you wont have to overwrite and undo as much and can more efficiently add your theme layer.
It is extremely light weight at only 20.6kb minified and gzipped. The goal is to keep it small and in return this will help keep your code small since you won’t really need to peel out the theme of the framework itself just to add your own look.
It is intended to be dropped into your sites similar to how you would a normalize.css file.