Move GTK theme from moongreet config to system-wide gtk-4.0 settings

Remove gtk-theme from moongreet.toml and set gtk-theme-name and
gtk-icon-theme-name in /etc/xdg/gtk-4.0/settings.ini instead.

defaults/etc/moongreet/moongreet.toml

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

defaults/xdg/gtk-4.0/settings.ini

@@ -2,4 +2,6 @@
 # ABOUTME: User overrides go in ~/.config/gtk-4.0/settings.ini
 
 [Settings]
+gtk-theme-name=Colloid-Grey-Dark-Catppuccin
+gtk-icon-theme-name=Colloid-Grey-Catppuccin-Dark
 gtk-application-prefer-dark-theme=1