I am still here. I did a new admin theme for the old CMS and I wanted to see how hard it is for the latest version. You can't create a new admin theme because... because it is hardcoded to the bootstrap3 name. But you don't need to. The actual theme works fine. There are just small issues when you want it worked different way...  

This is not something I would recommend for using, a better idea is to improve the core itself. This is just an example of what is possible.