Hello and thanks for answering me:
The first string was an attempt to write HTML tags inside an ECHO command, including PHP code.
In any case, of course I created and activated a child theme and I did what you suggested but, unfortunately, it doesn’t work. Have you tried on a local wp install?
Thanks in advance for your support. Best Regards.
EDIT: It works! 🙂 thanks Olga for your precious tips!