fix(themes): align night theme default with XML preference

Author: criticalAY

AnkiDroid/src/main/java/com/ichi2/anki/settings/Prefs.kt

@@ -362,7 +362,7 @@ open class PrefsRepository(
 
     val appTheme: AppTheme by enumPref(R.string.app_theme_key, AppTheme.FOLLOW_SYSTEM)
     val dayTheme: DayTheme by enumPref(R.string.day_theme_key, DayTheme.LIGHT)
-    val nightTheme: NightTheme by enumPref(R.string.night_theme_key, NightTheme.DARK)
+    val nightTheme: NightTheme by enumPref(R.string.night_theme_key, NightTheme.BLACK)
 
     //endregion