gnu: xfce: Use "Greybird" and "elementary-xfce-dark" as default theme.

* gnu/packages/patches/xfce4-settings-defaults.patch: Set "ThemeName"
to "Greybird", "IconThemeName" to "elementary-xfce-dark".
* gnu/packages/xfce.scm (xfce)[inputs]: Add elementary-xfce-icon-theme.

gnu/packages/patches/xfce4-settings-defaults.patch

@@ -1,17 +1,18 @@
-This patch make Xfce use "gnome" as the default icon theme and enable font antialias.
+This patch make Xfce use "elementary-xfce-dark" as the default icon theme, "Greybird" as
+the default GTK theme and enable font antialias.
 
-Taken from ArchLinux.
 
-diff -upr xfce4-settings-4.12.0.orig/xfsettingsd/xsettings.xml xfce4-settings-4.12.0/xfsettingsd/xsettings.xml
---- xfce4-settings-4.12.0.orig/xfsettingsd/xsettings.xml	2015-02-24 03:33:11.000000000 +0200
-+++ xfce4-settings-4.12.0/xfsettingsd/xsettings.xml	2015-03-01 09:52:24.376869688 +0200
+diff -upr xfce4-settings-4.18.2.orig/xfsettingsd/xsettings.xml xfce4-settings-4.18.2/xfsettingsd/xsettings.xml
+--- xfce4-settings-4.18.2.orig/xfsettingsd/xsettings.xml	2023-02-11 16:55:53.000000000 +0800
++++ xfce4-settings-4.18.2/xfsettingsd/xsettings.xml	2023-05-14 19:20:43.919701586 +0800
 @@ -6,8 +6,8 @@
- <?xml version="1.0" encoding="UTF-8"?>
+ 
  <channel name="xsettings" version="1.0">
    <property name="Net" type="empty">
-     <property name="ThemeName" type="empty"/>
+-    <property name="ThemeName" type="empty"/>
 -    <property name="IconThemeName" type="empty"/>
-+    <property name="IconThemeName" type="string" value="gnome"/>
++    <property name="ThemeName" type="string" value="Greybird"/>
++    <property name="IconThemeName" type="string" value="elementary-xfce-dark"/>
      <property name="DoubleClickTime" type="int" value="400"/>
      <property name="DoubleClickDistance" type="int" value="5"/>
      <property name="DndDragThreshold" type="int" value="8"/>

gnu/packages/xfce.scm

@@ -1176,9 +1176,7 @@ for and start applications.")
      (list exo
            garcon
            adwaita-icon-theme
-           ;; Greybird is the recommend theme since Xfce 4.14, see:
-           ;; <https://mail.xfce.org/pipermail/xfce4-dev/2018-August/032151.html>
-           ;; TODO: Make it default via "xfconf/xfce/perchannel-xml/xsettinsg.xml"
+           elementary-xfce-icon-theme
            greybird-gtk-theme
            hicolor-icon-theme
            mate-polkit-for-xfce