The activation must be done at Joomla backend:

Of course. Just as everything in the construction of the site. But WHERE please ?????????

Finaly, I found these fields in the template admin.

Great and thanks for info

Problem solved.

I appreciate your sense of humour (and your templates), but I didn't notice that I could enable the background videos in the template admin, because, at first, I had the free X2 template, without the full size videos. Then, when I installed the X2 pro, I didn't check once more the template admin.
So, if you just answered "Just upload your video as shown and activate the full size video.... in the template admin", my problem was immediately solved.

Anyway, thanks for your help, on saturday.