To add your own stylesheet or CSS to your S-Docs templates, you can edit the source of your templates. All you need to do is click on the Source button in the top left of the template editor. You can then copy your own <style></style> tags at the start of the page (be sure to use all lowercase and close your tags). In-line styles also work great and can be used throughout your template.
Not all styles, fonts and borders render precisely in all output formats, especially PDFs. At this time, you cannot include a separate CSS file; however, you can create a separate template with a separate CSS style and insert it into your current template as a merge field. In addition, the S-Docs package does include one stylesheet file as a static resource that you can leverage.