I don't use responsive classic, because I prefer the freedom to write code within the zen cart CSS rules with my own CSS rules and I found responsive classic to be a little clunky. I'd suggest you are looking at the minified CSS file for responsive classic as well. If you know what your doing you should be able to take the template_default originally written years ago and update it with your own HTML5 and CSS3 rules to make it a responsive template. I have a template that already does this and is written for various screen sizes which I use as a base for all new Zen Cart projects.
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