I think everyone needs to spend a year living in the other hemisphere so they know to never again announce something on the Internet as 'happening in summer' or 'ready this holiday'.
'Open-Source' appears six times on the posted page (source). With a properly versioned web page I would have expected it would be easy to publish that as 'Soon-to-be-open-source' and change that to 'Open-source' when it actually is. (Usual ambiguities about what 'open source' actually means to one side.)
Having said that, it does look like a lovely interface, and a good way to manage static sites -- though without a GNU/Linux release I won't be able to try it out.
Apparently the source code comes with the app:
From Bob:
"Hi Guys,
Publii is built with Electron, so the source code is available, you can read it directly after installing the app. Here is where the files are located: ;https://getpublii.com/docs/install-publii.html
We know that there is no source code repository, we will publish it soon, but before that we have to finish work on the implementation some key features and polish the code.
The next release will bring a dev documentation with the Starter theme, I hope this will help you to better understand app and find out how the themes are built."
https://publii.ticksy.com/ticket/1216232/
From Bob:
"We still preparing the dev documentation and I think it will be ready this holiday. Right now you can browse the themes packages and see how they are built. Below you can download the .pdf with handlebars tags used in Publii themes.
https://drive.google.com/file/d/0B92PpHk85ShpZk50NVFVQlZjX3c...
Referring to repository question, we will need to reflect further on this matter, but probably the GitHub is the best solution for that."