Question

How can I apply the Theme without restarting the whole App? If I do it with startActivity(getIntent()); finish(); the Activity quits and dont restart. Is it possible to simply restart / recreate the Activity to apply the theme?

Answer 1

It is in the incorrect order.

    finish();
    intent = new Intent(this, <your_activity>.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP); 
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(intent);

This is the correct order.

Theme can be set before super.onCreate(savedInstanceState); is being called. You need to destroy the activity and create it again and call immediately setTheme(THEME); in onCreate()