How to save and apply theme settings in an Android…

There are several approaches to save and apply theme settings in an Android application.

SharedPreferences: A simple way to save small amounts of data, such as the identifier of the selected theme.

// Saving theme
val sharedPref = activity?.getPreferences(Context.MODE_PRIVATE) ?: return
with (sharedPref.edit()) {
    putInt(getString(R.string.saved_theme_key), themeId)
    apply()
}

// Reading theme
val sharedPref = activity?.getPreferences(Context.MODE_PRIVATE)
val defaultValue = resources.getInteger(R.integer.default_theme_id)
val savedThemeId = sharedPref?.getInt(getString(R.string.saved_theme_key), defaultValue) ?: defaultValue

// Applying theme in Activity
setTheme(savedThemeId)
super.onCreate(savedInstanceState)

DataStore: A more modern and asynchronous way to save data, offering data streams and more reliable storage.

// DataStore
private val Context.dataStore: DataStore<Preferences> by preferencesDataStore(name = "settings")

// Saving theme
suspend fun saveTheme(themeId: Int) {
    dataStore.edit { settings ->
        settings[intPreferencesKey("theme_id")] = themeId
    }
}

// Getting theme
val themeFlow: Flow<Int> = dataStore.data
    .catch { exception ->
        if (exception is IOException) {
            emit(emptyPreferences())
        } else {
            throw exception
        }
    }
    .map { preferences ->
        preferences[intPreferencesKey("theme_id")] ?: R.style.Theme_App
    }

Applying the theme in an Activity can happen after retrieving the value from DataStore (for example, using collect in a coroutine) and calling setTheme. Recreating the Activity might be necessary after changing the theme for proper display.

System theme settings: Android also provides built-in support for themes: light, dark, and default (follows system settings). This can be implemented through resources.
- Creating styles for light and dark themes (e.g., res/values/themes.xml and res/values-night/themes.xml).
- Setting AppCompatDelegate.setDefaultNightMode to apply the theme programmatically or configuring it in the activity's manifest via android:theme.

Applying a theme often requires recreating the Activity to ensure resources are loaded correctly. You can use recreate() for this purpose.

How to save and apply theme settings in an Android application?