I tried to change the codes in /usr/share/icons/default/index.theme and i found that may be i deleted it somehow. is it the reason behind it? if so how can i restore it and make the option in gnome tweak tool active again?
I checked the packages for the directory
/usr/share/icons, then I reinstalled the package
x-cursor-themes by running
sudo apt install --reinstall xcursor-themes
and I got the file back.