If any one delete the Default theme (theme name is Default) then other themes will not work properly until you set another default theme using dconf editor or gsettings command .
use your theme name to replace USE-YOUR-THEME-NAME
gsettings set org.gnome.desktop.interface gtk-key-theme 'USE-YOUR-THEME-NAME'
for example you can try this
gsettings set org.gnome.desktop.interface gtk-key-theme 'Default1'
revert back
gsettings set org.gnome.desktop.interface gtk-key-theme 'Default'