If you are looking for PrintCSS templates to use or build upon, you should look at the newly published PrintCSS templates by DocRaptor. They are, of course, optimized to be rendered with DocRaptor or Prince, but with little adjustments, these templates should also render nicely with the other tools!
The templates cover various use cases, from resumes, invoices, and restaurant menus to packing slips.
The cool thing is these templates are released under the MIT license so that you can use them for personal and commercial projects! The whole code is also available on GitHub for you to fork or suggest improvements, and you may even want to submit a pull request with your suggestions!
I hope you all enjoy these templates and if you use them for your projects, let me know!