Thanks Ofi. I'm fighting with the parameters in this "Advanced" tab, but I don't know what is best , absolute or relative, and what I get in the visitors frontend is not what I have in the frontend editor.
Last minute : I think I have to give up, and use another SPPB addon. It's just a nightmare to make each element to display properly, and then to be responsive. After I had a display almost correct on large screens, I was trying to have an acceptable display on tablets. Impossible. I have overlayed elements, and I can't reach the parameters of the elements behind.
SPPB has a lot a possibilities, but some of them are not usable, and make us lose much time.