There is not documentation on theme designing at https://docs.evencart.com/evencart-themes/.
We are still working on creating theme documentation. But if you have ever used liquid (it's damn easy, believe me), then it should not be difficult for you to design a new theme. I think the best approach would be copy the existing theme - the default one, and change it. That way you'd be sure of the pages and their structure.
Hope that helps.