This was the first theme with the parent/child concept. Now, with changes in 2.3.1, this concept is most appealing now... You can't use the plugin anymore for extending your theme and working with just one set of theme shortcodes is not so effective.   

I fixed AppStrap4 and the existing child theme for 2.3.1. 

All information is here:  

Because this is a premium theme, each user gets private repo for its child theme. 

How to get this theme?  

1. Contact me, you should have GitHub and gitter account  

2. Select the type of homepage you want to have for your own theme here

3. The rest depends on our agreement - time, payment, access to repo, demo etc. 

Each child theme is custom work, so it takes time.