I've been tweaking my system theme lately and I have noticed that variables at the beginning of gtk-3.0/gtk.css don't change anything at all. Moreover all css rules later in file have hard-coded color values. Why is that? If these files were autogenerated there should be notice about tool used in README.md. Maybe porting all of that to native CSS variables
var(--name) is a good idea (if GTK supports that)?