Add gtk-theme to moongreet config for greeter session

The greeter runs as the greeter user, so user-level gsettings
don't apply. Explicit gtk-theme ensures Colloid-Grey-Dark-Catppuccin
is used in the login screen.

defaults/etc/moongreet/moongreet.toml

@@ -3,6 +3,7 @@
 
 [appearance]
 background = "/usr/share/moonarch/wallpaper.jpg"
+gtk-theme = "Colloid-Grey-Dark-Catppuccin"
 
 [behavior]
 # show_user_list = true