Remove the ugly border in Gnome 3.20

Here is a .config/gtk3.0/gtk.css file where the ugly border is removed. If there is no such file, just create it.

window.ssd headerbar.titlebar {
  border: none;
  background-image: linear-gradient(to bottom, 
    shade(@theme_bg_color, 1.05), 
    shade(@theme_bg_color, 0.99));
  box-shadow: inset 0 1px shade(@theme_bg_color, 1.4);
}