After Xfce 4.20 release set theme "Greybird" explicitely

Change theme to "Greybird" explicitely. It was used as the default theme in
postmarketOS Xfce4 already, but upstream changed the default to "Adwaita" [1].

[1] https://gitlab.xfce.org/xfce/xfce4-settings/-/commit/d1a04d48c1a78805faa530e453e3c0733944d025

config/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml

@@ -2,6 +2,7 @@
 
 <channel name="xsettings" version="1.0">
   <property name="Net" type="empty">
+    <property name="ThemeName" type="string" value="Greybird"/>
     <property name="IconThemeName" type="string" value="Faenza-Ambiance"/>
     <property name="DoubleClickDistance" type="int" value="20"/>
   </property>