Depends on how you want to manage the stylesheets. The ones in your image above the editor, are part of the Corporate Site theme, so those stylesheets physically exist in the file system. IIS will automatically include them in your site. There are good and bad with that approach. I personally use the UI for all my stylesheets and add or remove them dynamically as needed.
If you want the same functionality as in your image without having to use the editor you've circled in red, you can simply create new stylesheets in the CSS Stylesheet application and in your main one you assign to your site, @import the other stylesheets into that main stylesheet.
Another approach is to create new stylesheets in the CSS Stylesheet application then on your master page template in Design view, add a HEAD HTML webpart and include those stylesheets.