I'm learning to set up a child theme; one step is to copy style.css and modify the header appropriately, and then to start playing with the code.
But the style.css that comes with Semantic doesn't have any code, just a header! Why is this? Do I have to work with all the css files in the stylesheets folder?
The only purpose of using a child theme is that you can have your own modifications to theme files which includes template files and styling files without affecting the original theme. Hence we have provided a starter child theme along with semantic theme. The header in the child-theme's style.css file instructs WordPress to load the original files from the semantic theme and then add the styling specified in "this" (child theme's style.css file) on top of it. So you don't have to work with css files in the stylesheets folder at all. Just write whatever CSS modifications you need into the child-theme's style.css file and they will be applied.
Hope this helped.