Correct way to reference CSS files in Design

What's the correct way to reference CSS files in your design? I thought that you should do the whole mysource_files/main.css thing (provided that you have the css files as a child of the design).

    


But that's not working. What have I done wrong?

This only works if the main.css was uploaded as an associated file of the design. If you're using the CSS File asset, you need to use a linked_css design area instead. Check the Design Manual for more info on that.