Hi,
I select the body and click on "+" within the "classes" group. I can enter my css class name.
But when I save the page, close the page and then re-open the page this class is no more there.
This seems to be a bug.
Hi e-mano-e
This is special tag, at this moment, you can't add class to it.
You can try to add a class for this page by going to menu item settings in backend and add a page class under Page Display tab: https://prnt.sc/u2kqr5
Regards