What the issue says in the title: If you don't have a gtk-dark.css file in the gtk-3.20 folder, it'll try to use gtk-3.0/gtk-dark.css instead for the application styling leading to a very broken application style. Even just a symlink to gtk.css in the name of gtk-dark.css in gtk-3.20 is enough to fix this.
What the issue says in the title: If you don't have a gtk-dark.css file in the gtk-3.20 folder, it'll try to use gtk-3.0/gtk-dark.css instead for the application styling leading to a very broken application style. Even just a symlink to gtk.css in the name of gtk-dark.css in gtk-3.20 is enough to fix this.