This patch series adds (gnu packages gnome-xyz) for GNOME themes[1] and
extensions[2].  I've initialized it with some packages that I
personally use.  Another potential candidate for inclusion into this
file would for instance be the Matcha GTK theme (bug#38095).

I'm not 100% sure on the licenses.  In some places, I only found
LICENSE files without a comment specifying "or later" elsewhere --
perhaps I've overlooked some.  For now, I'm using the stricter versions
(sans "+").

[1] https://www.gnome-look.org/
[2] https://extensions.gnome.org/