Android :: How to change theme from other apk programmatically

You would need to use the application's resources to load the theme. I have answered this in another thread here: https://stackoverflow.com/a/41948943/1048340

Here is an example where the package "com.example.theme" is installed and we use the app's resources and theme style in another app:

java

public class MainActivity extends AppCompatActivity {

  Resources resources;

  @Override protected void onCreate(Bundle savedInstanceState) {

    int themeResId = getResources().getIdentifier("AppTheme", "style", "com.example.theme");
    if (themeResId != 0) {
      setTheme(themeResId);
    }

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
  }

  @Override public Resources getResources() {
    if (resources == null) {
      try {
        resources = getPackageManager().getResourcesForApplication("com.example.theme");
      } catch (PackageManager.NameNotFoundException e) {
        resources = super.getResources();
      }
    }
    return resources;
  }

}

See here for a working project on GitHub.

This can lead to problems because all layouts, drawables, strings, etc. will be loaded from the other application's resources. Therefore, you should avoid using a theme from another package and instead copy the resources and theme to your own project.