It looks hard, but it's really easy to start to use GitHub as Postachio.io theme editor. Here's my step-by-step guide.
This is brief translation of my original post
Connect Postach.io and GitHub
The first and obvious, you must register on GitHub.
Then you should fork any of Postach.io's theme on Github. I have problems with clean fork, but everything works fine with clone.
Choose any theme and click Fork:
After this you'll get new repository. You can get access to all your repositories on your Profile page, Repositories tab:
You need to go to Source Code page inside Postach.io's settings. Connect with GitHub and choose repository from previous step:
Click Save and you can enjoy new theme.
Make your own theme
Everything is easier if you want to use your own theme. You need to create new repository. You can do it on GitHub Profile page, just click New button:
Choose name, description (if you want), choose "Public, activate Initialize this repository with a README" checkbox (if you want). This file will be useful for collaboration work. Then click Create Repository.
There are many-many letters on next step... Just click Import Code and import any existing repository.
Next page is Import page. What repository should you choose? If you need empty .html file where you can create new theme from scratch, then use this link
and click Begin Import
Then Continue to Repository:
Then you should see this page:
All code of your future theme should be in theme.html. You should see this file after fork or clone Postach.io's themes. Or you can create one.
Create your code
To edit code of your theme just click theme.html on your repository page and you'll see an editor:
In my case this file is empey 'cause I didn't create any code. To edit your code click on pencil icon and start coding:
Then click Commit Changes and check them on your blog. Don't forget to connect Postach.io with your repository.
You can ask any questions about Postach.io in comments. I'm always glad to help you