Okay! Finally I have found the solution! (even if I don’t understand why we have to do this that way).
In fact we have not to modify the function.php file but the customizer.php with the code I quoted above (in the question).
Now I really feel released from that issue, but anyone could explain me why is it that file that we have to modify? I think I haven’t really understood the role of the differents files.
The inc subdirectory
The inc subdirectory content
