x11-themes/zuki-themes: add new port

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.
• xfwm4 contains the theme that styles the xfwm4 window manager for XFCE4.

https://github.com/lassekongo83/zuki-themes
(cherry picked from commit 53b4ed309e3be33871aac5e3009414aba10ae857)