QUESTIONS

Just add this into your apps.
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AppDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES) 
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u can use :
MODE_NIGHT_NO
MODE_NIGHT_AUTO
MODE_NIGHT_FOLLOW_SYSTEM

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Change your theme to:
Theme.AppCompat.DayNight

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Create a folder named value-night and create styles.xml file and customize your night color inside.

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Your night mode reset every time you close your app. Use shared preference to store value and load your theme every time your app run..
To check status, u can use :
int mode = AppCompatDelegate.getDefaultNightMode();

you may use setTheme + recreae() for that, then add sharedpreferences to store changes