Fragment Background is Transparent

Question

I have a Fragment with a RecyclerView that displays items. When I click on one item a new Fragment is instantiated:

public void onItemClick(View view, int position) {
    getActivity().getSupportFragmentManager().beginTransaction()
            .setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN)
            .add(R.id.detail_container, new DetailFragment())
            .addToBackStack(null)
            .commit();
    selected_item = position;
}

and added to a container with a black background:

<FrameLayout
    android:id="@+id/detail_container"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="30dp"
    android:background="@android:color/black" />

However as you can see the background is still transparent.. I tried a lot of different approaches but they didn't worked..