The Stylesheet template value is one place you can use to define additional styles for your artboard.
The template is empty by default but if you add your own styles be sure to include the styles_content token before or after your content to make sure the page contents are included.
Example stylesheet content
There is a special token called styles_content that is used to place the write the page styles.
If you exclude this token your CSS will be written but the page CSS will not be written.