Well, finally, the theme is not to blame : I made a few more tests yesterday, and the problem only occurs in my local test installation (Window 10, wamp 3.0.6, wordpress 4.6.1) ! In my target environment (hosted on the internet with a professional wordpress install on linux), everything works perfectly with all the themes I tested.
Do you know if I need some specific configuration or extension to make in work on my windows environment ?
Thanks for your help ! ??