code: public static void apply(@NonNull Activity activity) {
          ViewGroup root = (ViewGroup) activity.getWindow().getDecorView();
          if (!root.isInEditMode()) {
              applyAllRecursively(root, activity.getAssets());
          }
      }
docs: Applies fonts to all TextView views in Activity's window decor view.<br/>
  TextView tag will be used to determine font.