Parishith Atleast nothing team can give support to change the color to white or black accordingly.
That’s up to each individual app and its developer(s) to enable edge-to-edge display for that app: https://developer.android.com/develop/ui/views/layout/edge-to-edge
So, to put it shortly, Nothing is not developing non-Nothing apps such as that browser on your photo.
Apps targeting Android 15 or higher will be forced to use it: https://developer.android.com/about/versions/15/behavior-changes-15#edge-to-edge
But you are on Android 15 correct, yes. Being on Android 15 is not enough, the focus is on the word targeting; so the app has to target Android 15 like I said before, and that won’t be required by the Google Play Store until August 31, 2025. After that it is mandated by Google.
Edit: Forgot to include this with extra explanation from the devs of Android themselves: