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. I created a new dashboard panel with a single change in the core file - to allow the dashboard with the name "dashboard" to be used new way. 

Originally it extended infopanel class but after digging deeper and finding that there is so old legacy code and answering "there is no time to fix it" - I did it and created a separate class without any legacy code. 

The priority was not to change core files too much. In fact, the result is the combination of infopanel and flexpanel functionality, only that left panel is stable. And everything is managed by one template file. 

Colors are set by global CSS variables so it can be customized in anybody's liking. Yeah, jmadmin plugin should be rewritten then. But there are important things. 

Those are examples:

And admin panel for admin with limited access: