A sample to give the option to switch the theme b/w the light and dark mode

arrays.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string-array name="theme_options_key">
        <item>Light</item>
        <item>Dark</item>
        <item>Set by battery saver</item>
    </string-array>
    <string-array name="theme_options_values">
        <item>light</item>
        <item>dark</item>
        <item>default</item>
    </string-array>
</resources>

arrays.xml (v29)

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string-array name="theme_options_key">
        <item>Light</item>
        <item>Dark</item>
        <item>System default</item>
    </string-array>
</resources>

MyApplication.kt

class MyApplication : Application(){
  
  override fun onCreate() {
        super.onCreate()
        applyTheme()
  }
  
  private fun applyTheme() {
        PreferenceManager.getDefaultSharedPreferences(this)
            .getString(getString(R.string.key_app_theme), ThemeHelper.DEFAULT_MODE)?.apply {
                // we need to re-apply the theme since the system does not saves the
                // information about our theme
                ThemeHelper.applyTheme(this)
            }
    }
}

preference_keys.xml

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:tools="http://schemas.android.com/tools" xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2" tools:ignore="MissingTranslation">
    <string name="key_app_theme"><xliff:g id="preference_key">selected_app_theme</xliff:g></string>
</resources>

preferences.xml

<?xml version="1.0" encoding="utf-8"?>
<PreferenceScreen xmlns:app="http://schemas.android.com/apk/res-auto">

    <ListPreference
        app:defaultValue="default"
        app:entries="@array/theme_options_key"
        app:entryValues="@array/theme_options_values"
        app:iconSpaceReserved="false"
        app:key="@string/key_app_theme"
        app:title="@string/title_choose_theme"
        app:useSimpleSummaryProvider="true" />
</PreferenceScreen>

SettingsFragment.kt

import android.os.Bundle
import androidx.preference.ListPreference
import androidx.preference.Preference
import androidx.preference.PreferenceFragmentCompat
import com.unifa_e.bus.driver.R
import com.unifa_e.bus.driver.common.ThemeHelper

class SettingsFragment : PreferenceFragmentCompat() {
    override fun onCreatePreferences(savedInstanceState: Bundle?, rootKey: String?) {
        setPreferencesFromResource(R.xml.preferences, rootKey)

        listenAppThemeChanges()
    }

    /***
     * applies the new theme whenever there is a change
     */
    private fun listenAppThemeChanges() {
        findPreference<ListPreference>(getString(R.string.key_app_theme))?.apply {
            onPreferenceChangeListener = Preference.OnPreferenceChangeListener { _, newValue ->
                ThemeHelper.applyTheme(newValue as String)
                true
            }
        }
    }
}

ThemeHelper.kt

import android.os.Build
import androidx.appcompat.app.AppCompatDelegate

object ThemeHelper {

    //the values must be same as defined in arrays.xml
    private const val LIGHT_MODE = "light"
    private const val DARK_MODE = "dark"
    const val DEFAULT_MODE = "default" // follow system settings

    fun applyTheme(theme: String) {
        val mode = when (theme) {
            LIGHT_MODE -> AppCompatDelegate.MODE_NIGHT_NO
            DARK_MODE -> AppCompatDelegate.MODE_NIGHT_YES
            else -> {
                // Android 10 and above supports Dark theme by default
                // For others, Battery saver mode switches the system to Dark theme
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
                    AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM
                } else {
                    AppCompatDelegate.MODE_NIGHT_AUTO_BATTERY
                }
            }
        }
        AppCompatDelegate.setDefaultNightMode(mode)
    }
}