You need to be on Android 6.0 to be able to flash custom ROMs.
Install this recovery: https://drive.google.com/file/d/0B1pErR_Eg7cXSGRycW9tZzNLd3c/view
After this press on the link below to install the stock ROM Android 6.0:
How to install:
1: Download any root explorer
2: Copy bootanimation.zip to: /system/media
3: Set the permission on 664/ rw-r-r
720 x 1280: Download...
You can do it if you are rooted. After you have root you can install an app called Layers on playstore then find a theme for it.
This should fix the proximity sensor if somebody has problems with this.
Could someone make a magisk module to play audio from earpiece? Like for example S8 and OnePlusOne has a mod like this.