Implementing user choice of theme

You can also change dynamically theme using:

ContextThemeWrapper w = new ContextThemeWrapper(this, <newTHEMEId>);
getTheme().setTo(w.getTheme());

Before onCreate for each activity.

I actually have this feature in my application and additionally, I allow users to change theme at runtime. As reading a value from preferences takes some time, I'm getting a theme id via globally accessible function which holds cached value.

As already pointed out - create some Android themes, using this guide. You will have at least two <style> items in your styles.xml file. For example:

<style name="Theme.App.Light" parent="@style/Theme.Light">...</style>
<style name="Theme.App.Dark" parent="@style/Theme">...</style>

Now, you have to apply one of these styles to your activities. I'm doing this in activitie's onCreate method, before any other call:

setTheme(MyApplication.getThemeId());

getThemeId is a method which returns cached theme ID:

public static int getThemeId()
{
    return themeId;
}

This field is being updated by another method:

public static void reloadTheme()
{
    themeSetting = PreferenceManager.getDefaultSharedPreferences(context).getString("defaultTheme", "0");
    if(themeSetting.equals("0"))
        themeId = R.style.Theme_Light;
    else
        themeId = R.style.Theme_Dark;
}

Which is being called whenever preferences are changed (and, on startup of course). These two methods reside in MyApplication class, which extends Application. The preference change listener is described at the end of this post and resides in main activity class.

The last and pretty important thing - theme is applied, when an activity starts. Assuming, you can change a theme only in preference screen and that there's only one way of getting there, i.e. from only one (main) activity, this activity won't be restarted when you will exit preference screen - the old theme still will be used. Here's the fix for that (restarts your main activity):

@Override
protected void onResume() {
    super.onResume();
    if(schduledRestart)
    {
        schduledRestart = false;
        Intent i = getBaseContext().getPackageManager().getLaunchIntentForPackage( getBaseContext().getPackageName() );
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        startActivity(i);
    }
}

scheduledRestart is a boolean variable, initially set to false. It's set to true when theme is changed by this listener, which also updates cached theme ID mentioned before:

private class ThemeListener implements OnSharedPreferenceChangeListener{

    @Override
    public void onSharedPreferenceChanged(SharedPreferences spref, String key) {
        if(key.equals("defaultTheme") && !spref.getString(key, "0").equals(MyApplication.getThemeSetting()))
        {
            MyApplication.reloadTheme();
            schduledRestart = true;
        }
    }


sp = PreferenceManager.getDefaultSharedPreferences(this);

listener = new ThemeListener();
sp.registerOnSharedPreferenceChangeListener(listener);

Remember to hold a reference to the listener object, otherwise it will be garbage colleted (and will cease to work).

If you are using Material Components themes and followed Light and Dark theme guidelines then you can do it from AppCompatDelegate. These themes can be changed/applied at run time without restarting your application.

private fun handleThemeChange(theme: String) {
        when (newTheme) {
            getString(R.string.light) -> AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO)
            getString(R.string.dark) -> AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_YES)
            getString(R.string.system) -> AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM)

        }
    }

It does work if you do it this way, and I don't think it would cause any problem, but it seems like a lot of hassle (you have to multiply all your layouts by all the themes you want to add. If later you want to modify a resource in a layout, you'll have to modify it in all the themes. You're definitely bound to forget one)

Why not using the Styles and Themes feature of Android?

They can be applied to the whole activity easily:

<activity android:theme="@style/my_theme">

So that when you detect a change in the SharedPreferences value you use (button on a preference activity, or something) you can just switch the style. Or better, you can set the style to read your preference value at runtime (when creating the activity) and apply the correct style/theme accordingly.