Currently there's a default theme handcrafted by a human, and a theme named "dark" generated by Claude 3.7 Sonnet as an example. These themes are there for your inspiration, you can make your own by copying the default folder and customizing the HTML and CSS to look and work the way you want.
Just remember to include the template variables, they look like and are how SyMark knows where to inject your converted text into the site template. These template variables get generated on a per-page basis, so they can be different depending on other context, an example of this is the table of contents, which generates different output based on the page its included in.