fix icon theme references, replace Newaita with Colloid-Catppuccin

- Update gtk-3.0/settings.ini icon theme to match gsettings
- Replace hardcoded Newaita icon path in moonarch-btnote with
  generic icon name (uses active theme automatically)
- Update README to reflect correct icon theme

defaults/xdg/gtk-3.0/settings.ini

@@ -3,7 +3,7 @@
 
 [Settings]
 gtk-theme-name=Colloid-Catppuccin
-gtk-icon-theme-name=Newaita-reborn-gray-dark
+gtk-icon-theme-name=Colloid-Grey-Catppuccin-Dark
 gtk-font-name=UbuntuSans Nerd Font 11
 gtk-cursor-theme-name=Sweet-cursors
 gtk-cursor-theme-size=24