gnu: Add greybird-gtk-theme.

* gnu/packages/gnome-xyz.scm (greybird-gtk-theme): New variable.

gnu/packages/gnome-xyz.scm

@@ -4,6 +4,7 @@
 ;;; Copyright © 2019 Giacomo Leidi <goodoldpaul@autistici.org>
 ;;; Copyright © 2020 Alex Griffin <a@ajgrf.com>
 ;;; Copyright © 2020 Jack Hill <jackhill@jackhill.us>
+;;; Copyright © 2020 Kei Kebreau <kkebreau@posteo.net>
 ;;; Copyright © 2020 Ekaitz Zarraga <ekaitz@elenq.tech>
 ;;; Copyright © 2020 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;; Copyright © 2020 Ryan Prior <rprior@protonmail.com>
@@ -42,6 +43,7 @@
   #:use-module (gnu packages ssh)
   #:use-module (gnu packages tls)
   #:use-module (gnu packages ruby)
+  #:use-module (gnu packages web)
   #:use-module (gnu packages xml))
 
 (define-public matcha-theme
@@ -515,6 +517,34 @@ scrollable tiling of windows and per monitor workspaces.  It's inspired by paper
 notebooks and tiling window managers.")
     (license license:gpl3)))
 
+(define-public greybird-gtk-theme
+  (package
+    (name "greybird-gtk-theme")
+    (version "3.22.12")
+    (source (origin
+              (method git-fetch)
+              (uri
+               (git-reference
+                (url "https://github.com/shimmerproject/Greybird")
+                (commit (string-append "v" version))))
+              (file-name (git-file-name name version))
+              (sha256
+               (base32
+                "1j66ddvl3pmwh2v8ajm8r5g5nbsr7r262ff1qn2nf3i0gy8b3lq8"))))
+    (build-system meson-build-system)
+    (native-inputs
+     `(("gtk+" ,gtk+)
+       ("glib:bin" ,glib "bin")         ; for "glib-compile-resources"
+       ("librsvg" ,librsvg)
+       ("pkg-config" ,pkg-config)
+       ("ruby-sass" ,ruby-sass)
+       ("sassc" ,sassc)))
+    (home-page "https://shimmerproject.org/")
+    (synopsis "Grey GTK+ theme based on Bluebird")
+    (description "Greybird is a grey derivative of the Bluebird theme by the
+Shimmer Project.  It supports GNOME, Unity, and Xfce.")
+    (license (list license:gpl2+ license:cc-by-sa3.0))))
+
 (define-public numix-gtk-theme
   (package
     (name "numix-gtk-theme")