No need for improvement here, Serge. Creating a form within a form within a form is just bad coding style. Holger, instead of TForm, use TFrame. That's what they are there for.
Thanks for the advice! The one with the frames would certainly be a solution.
However, child forms should already be possible. This is currently not possible with TsSkinProvider on any other form (RX2, Vers. 13.02 Beta). In version 10.13 it still works, even without TsSkinProvider!