MainActivity.kt

class MainActivity : AppCompatActivity() {

    var selection: String? = null
    var checked_item = 0
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        
        val theme = applicationContext.resources.getStringArray(R.array.theme)
        button.setOnClickListener{
            val builder = AlertDialog.Builder(this)
            builder.setTitle("Choose Theme")
            builder.setSingleChoiceItems(R.array.theme,SaveSharedPrefernce.getChecktheme(applicationContext))
            { dialog, which -> selection = theme[which]
                checked_item = which
            }
            builder.setPositiveButton("OK"){dialog, which ->
                if (selection==null){
                    checked_item = SaveSharedPrefernce.getChecktheme(applicationContext)
                    selection = theme[checked_item]
                }
                when(selection){
                    "System Default"->AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM)
                    "Dark"->AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES)
                    "Light"->AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO)
                }
                SaveSharedPrefernce.setChecktheme(applicationContext,checked_item)
            }
            builder.setNegativeButton("Cancel"){dialog, which -> dialog.dismiss() }
            val diag = builder.create()
            diag.show()
        }
    }
}