Where do the themes come from?

Stastic provide a list of themes for you to create a new site on Stastic - checkout the section “[Stastic installer: choose between “new website” and “existing website”]”:

Choose a theme to create a website for free with Stastic editor

These themes are open source themes, provided for free by the Jekyll community. In the list you will find a link to each theme source and author.

Also you might be interested in the section “[Why so few themes?]”.