Simple gray themes for GTK-based desktops.

Features:
- gnome-shell is the theme for GNOME Shell. This themes stuff like the
  calendar widget, the panel, the dock and more.
- gtk-2.0 contains the GTK+2 theme, styling applications that doesn't
  use GTK+3 yet.
- gtk-3.0 contains the GTK+3 theme, styling most modern GTK+ applications.
- gtk-4.0 contains the GTK+4 theme, styling most modern GTK+ applications.
- xfwm4 contains the theme that styles the xfwm4 window manager for XFCE4.