code: public static SearchView setBackground(SearchView view, @DrawableRes int drawableId) {
          int id = view.getResources().getIdentifier("android:id/search_plate", null, null);
          if (id > 0) {
              View search = view.findViewById(id);
              if (search != null) {
                  search.setBackgroundResource(drawableId);
              }
          }
          return view;
      }
docs: Override the default background with a theme version.