create-highcontrast: update usage message
create-highcontrast: update usage message

gnome-icon-theme-symbolic is no more

https://bugzilla.gnome.org/show_bug.cgi?id=740444

Distrotech/gnome-themes-standard,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,DoctorJellyface/gnome-themes-standard