Creating Child Theme from "Blend"

I am using the theme “Blend” and want to create a child theme to contain my modifications. The styles in Blend are pulled from a file called style.cfg, which is located in the folder for whatever subtheme you are using (“Coffee” in this case). Changes to the style.css file do not show up; you have to modify style.cfg. I have successfully created a new .cfg file and had the template call that instead, but it requires changing css.php, and my changes would be lost in an update.

My question is, when I set up a child theme, how do I set it up so that a new style.cfg file overrides the one in the original theme? Alternately, could I have a child css.php so that overrides the original and calls a different, modified file?

Thanks!