This is a subclass of GtkButton special theming for the close button. taken from: https://git.gnome.org/browse/gnome-terminal/commit/src?h=gnome-3-8&id=c3a3e06 https://git.gnome.org/browse/gnome-terminal/diff/src/terminal-close-button.c?h=gnome-3-8&id=2bff4b6