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/bin/moonarch-btnote

@@ -3,7 +3,7 @@
 # ABOUTME: Can be run periodically via timer or cron.
 
 NOTIFY_AT_PERCENTAGE=70
-ICON="/usr/share/icons/Newaita-reborn/status/symbolic/battery-empty-symbolic.svg"
+ICON="battery-empty"
 
 for d in $(upower -e); do
   DEVICE_DATA=$(upower -i "$d")