docs: add default theme customer guide

Author: SayanthRock

docs/DEFAULT_THEME_CUSTOMER_GUIDE.md

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+# Default Theme Customer Guide
+
+Use this guide when sharing the default theme with customers.
+
+## Customer steps
+
+1. Download the latest module ZIP.
+2. Install it using the supported root manager if the module version is required.
+3. Reboot the phone.
+4. Apply wallpaper from normal wallpaper settings.
+5. Test home screen and lock screen separately.
+6. Send a device report if something does not work.
+
+## Default theme options
+
+| Option | Customer action | Status |
+|---|---|---|
+| Home wallpaper | Apply from wallpaper settings | Safe |
+| Lock wallpaper | Apply from wallpaper settings | Safe where supported |
+| Theme folder | Install module ZIP | Test required |
+| Icons | Use launcher or OEM theme support | Test required |
+| Sounds | Apply manually if supported | Test required |
+| Support report | Send device and screenshot details | Safe |
+
+## Cool improvement ideas
+
+- Add 4K dark premium wallpapers
+- Add AMOLED black wallpaper set
+- Add Desert Sand glow wallpaper set
+- Add lock screen clock preview images
+- Add home screen widget preview images
+- Add before and after screenshots
+- Add device wise support badges
+- Add customer install video later
+
+## Customer promise
+
+Give customers clear options and real tested support. Do not mark a feature stable until it is tested on that device model and firmware.