☆ Thank you very much for answering
I am going to have a serious look to Themer (I downloaded it yesterday but yet not given it a try, I am a lit bit afraid...).
It's that I am becoming crazy ♪ a little ...
I have modified antimatter theme to have it the way I wanted for blog purposes: it's perfect but I would also like to have a completly different page for welcoming people at first, make a presentation... with a full width picture as background and presentation text on it (see mediator as @rhuster says in forum to someone asking about that)
So far, I have tried to have a modular page from antimatter, but with the modifications I have in base.html.twig makes it complicated.
I am not able yet to understand how to adapt base.html.twig to make it correspond to what I want ...