Unfortunately, I'm still working locally right now. But the problem seems pretty obvious to me:
With the page body selected, I delete the 'jpb-page' class in the style manager. After saving and reopening the editor, the class is there again. With this class, unfortunately, styles of the base template are overwritten.
I also tried to change the default classes for columns or containers. For example, I deleted the class 'col' and inserted 'col-md-3' instead. After reopening the page the default class 'col' was there again ('col-md-3' as well) and overwrote the intended column behavior.
I looked around a bit in the T4 Page Builder PHP code and found 2 files where the 'jpb-page" is hard assigned.
.\components\com_t4pagebuilder\views\page\tmpl\default.php
.\plugins\content\loadt4pagebuilder\loadt4pagebuilder.php
I have not been able to find a neat clean solution to this so far, so I have discarded T4 Page Builder at the moment....
But I would love to use it, it's a fantastic piece of software!
Thanks a lot!