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

    <application ...>
        <activity
            android:name=".MainActivity"
            android:exported="true"
            android:label="@string/app_name"
            android:theme="@style/Theme.DarkMode"
			android:configChanges="uiMode">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

class MainActivity : ComponentActivity() {
    ...
    override fun onConfigurationChanged(newConfig: Configuration) {
        super.onConfigurationChanged(newConfig)

        val _nightMode = newConfig.uiMode and Configuration.UI_MODE_NIGHT_MASK
        when(_nightMode) {
            Configuration.UI_MODE_NIGHT_NO -> {/* 処理 */}       // Night Mode
            Configuration.UI_MODE_NIGHT_YES -> {/* 処理 */}      // Day Mode
            Configuration.UI_MODE_NIGHT_UNDEFINED -> {/* 処理 */}
            else -> {}
        }
    }
	...
}

    /**
     * Activity Action: Show settings to allow configuration of display.
     * <p>
     * In some cases, a matching Activity may not exist, so ensure you
     * safeguard against this.
     * <p>
     * Input: Nothing.
     * <p>
     * Output: Nothing.
     */
    @SdkConstant(SdkConstantType.ACTIVITY_INTENT_ACTION)
    public static final String ACTION_DISPLAY_SETTINGS =
            "android.settings.DISPLAY_SETTINGS";

    fun openSettingsDisplay() {
        startActivity(
            Intent(Settings.ACTION_DISPLAY_SETTINGS)
        )
    }

    /**
     * Activity Action: Show settings to allow configuration of Dark theme.
     * <p>
     * In some cases, a matching Activity may not exist, so ensure you
     * safeguard against this.
     * <p>
     * Input: Nothing.
     * <p>
     * Output: Nothing.
     *
     * @hide
     */
    @SdkConstant(SdkConstantType.ACTIVITY_INTENT_ACTION)
    public static final String ACTION_DARK_THEME_SETTINGS =
            "android.settings.DARK_THEME_SETTINGS";

    @RequiresApi(Build.VERSION_CODES.R)
    fun openSettingsDarkTheme() {
        startActivity(
            Intent("android.settings.DARK_THEME_SETTINGS")	// 文字列を直書き
        )
    }