How to switch themes (night mode) without restarting the activity?

for those who are trying to find solution for android version 10 or updated.

to set dark/light mode use this:

AppCompatDelegate.setDefaultNightMode(state) //state can be AppCompatDelegate.MODE_NIGHT_YES or AppCompatDelegate.MODE_NIGHT_NO

it will change the display of your app but with a flicker

to avoid the activity recreation flicker (for smooth transition), in your activity add the below method

@Override
    public void recreate() {
        finish();
        overridePendingTransition(R.anim.anime_fade_in,
                                  R.anim.anime_fade_out);
        startActivity(getIntent());
        overridePendingTransition(R.anim.anime_fade_in,
                                  R.anim.anime_fade_out);
    }

@Alexander Hanssen's answer basically has answered this... Don't know why it was not accepted... Maybe because of the finish()/startActivity(). I voted for it and I tried to comment but cannot...

Anyway, I would do exactly what he described in terms of styles.

<style name="AppThemeLight" parent="Theme.AppCompat.Light">
    <!-- Customize your theme here. -->
    <item name="android:windowAnimationStyle">@style/WindowAnimationTransition</item>
</style>
<style name="AppThemeDark" parent="Theme.AppCompat">
    <!-- Customize your theme here. -->
    <item name="android:windowAnimationStyle">@style/WindowAnimationTransition</item>
</style>
<!-- This will set the fade in animation on all your activities by default -->
<style name="WindowAnimationTransition">
    <item name="android:windowEnterAnimation">@android:anim/fade_in</item>
    <item name="android:windowExitAnimation">@android:anim/fade_out</item>
</style>

But instead of finish/start with new intent:

Intent intent = new Intent(this, <yourclass>.class);
startActivity(intent);
finish();

I would do:

@Override
protected void onCreate(Bundle savedInstanceState) {

    // MUST do this before super call or setContentView(...)
    // pick which theme DAY or NIGHT from settings
    setTheme(someSettings.get(PREFFERED_THEME) ? R.style.AppThemeLight : R.style.AppThemeDark);

    super.onCreate(savedInstanceState);
}

// Somewhere in your activity where the button switches the theme
btn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {

        // decide which theme to use DAY or NIGHT and save it
        someSettings.save(PREFFERED_THEME, isDay());

        Activity.this.recreate();
    }
});

The effect is as shown in the video...

There isn't anything preventing you from calling setTheme() and then setContentView() again. You'll just need to restructure your app a bit so that, if you change the theme, you need to reinitialize any member variables you might have that are holding references to View objects.

The transition/animation makes the theme change seamless when you restart the activity, and this can be done by adding the items "android:windowanimationStyle" to your themes, and then referencing a style where you specifiy how the Activity should animate when it enters and exits. Note that this makes the animation apply on all activities with that theme.

<style name="AppThemeLight" parent="Theme.AppCompat.Light">
    <!-- Customize your theme here. -->
    <item name="android:windowAnimationStyle">@style/WindowAnimationTransition</item>
</style>
<style name="AppThemeDark" parent="Theme.AppCompat">
    <!-- Customize your theme here. -->
    <item name="android:windowAnimationStyle">@style/WindowAnimationTransition</item>
</style>
<!-- This will set the fade in animation on all your activities by default -->
<style name="WindowAnimationTransition">
    <item name="android:windowEnterAnimation">@android:anim/fade_in</item>
    <item name="android:windowExitAnimation">@android:anim/fade_out</item>
</style>

Then, when you want to change theme you could do this when clicking a button:

AppSettings settings = AppSettings.getInstance(this);
settings.set(AppSettings.Key.USE_DARK_THEME,
!settings.getBoolean(AppSettings.Key.USE_DARK_THEME));
Intent intent = new Intent(this, <yourclass>.class);
startActivity(intent);
finish();

Then in your onCreate method, use the setTheme() to apply the theme that is currently set in AppSettings like this:

AppSettings settings = AppSettings.getInstance(this);
setTheme(settings.getBoolean(AppSettings.Key.USE_DARK_THEME) ? R.style.AppThemeDark : R.style.AppThemeLight);
super.onCreate(savedInstanceState);
setContentView(<yourlayouthere>);

Check out this gist for reference: https://gist.github.com/alphamu/f2469c28e17b24114fe5