Thanks Saguaros for your interest.
I've found finally the solution exploring the console with all the cache, the minifiing, the optimize..etc...turned OFF and with the Template in developer mode.
In developer mode the Media Browser "modal" partially opens and I can see some warnings.
- Deprecated: The behavior of unparenthesized expressions containing both '.' and '+'/'-' will change in PHP 8: '+'/'-' will take a higher precedence in...
- and problem of variables inside a "toolbar" folder. (Toolbar folder deleted from bs3 and resolved)
I've seen in the console that the CSS is read from files .less.css so I have had the idea to PUSH THE BUTTON "LESS to CSS" in Purity toolbar. et voilà, the CSS now rewritten in my template folder is working, not totally YET but is working now opens!
A little customization at row 4119 and the modal is now perfect.
.j40 .modal-body,
.j42 .modal-body {
overflow: hidden;
padding: 0;
height: 80vh;
}
I hope this will be useful for someone!
Ciao
Saluti
Erik