In this example, it seems that the reason for creating a Styles.css file in the root directory of the web site was because the formating defined is supposed to be applied to both of the themes.
Styles.css is just an arbitrary name / locatiion. You can pick whatever makes sense for you. To keep the themes self-contained, you could duplicate the code in each of the theme's CSS files. However, by putting it in a shared file, you only need to manage the code once for multiple themes.