Though this is tested on OP6T, it could be working for most Android Oreo and later phones.
So I was annoyed not having an LED notification light on OnePlus 6T so I tried to make one of my own.
As OP6T has amoled display whenever a desired notification comes the app brings in a black screen with notification LED looking animation.
To use it:
1. Disable Ambient Display and any battery optimisation for the app.
2. Give notification access permission.
3 Select apps you want notifications for, by choosing a color (Don't yet select "Phone" app as it has bugs currently).
Note: On successfully selecting a color for an app the text in which the App name is written should change to that color
If having problem head to : https://forum.xda-developers.com/oneplus-6t/themes/app-amoled-notification-light-t3943715/post79810512#post79810512
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