<Spinner
  android:id="@+id/myspinner"
  android:layout_width="fill_parent"
  android:layout_height="wrap_content"
  android:background="@null"
  android:drawSelectorOnTop="true" />

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:background="@drawable/list_selector"
  android:paddingBottom="@dimen/padding_medium"
  android:layout_marginBottom="@dimen/padding_medium"
  android:orientation="vertical">

  ..........

</RelativeLayout>

<selector xmlns:android="http://schemas.android.com/apk/res/android">
  <!-- Pressed -->
  <item
    android:state_pressed="true"
    android:drawable="@color/green" /> <!--  @drawable/tab_press -->

  <!-- Selected -->
  <item
    android:state_selected="true"
    android:drawable="@color/green" /> <!--  @drawable/tab_press -->

</selector>

<style name="AppTheme" parent="android:Theme.Light">
     <item name="android:background">#000000</item>
     <item name="android:textColor">#FFFFFF</item>
     <item name="android:typeface">sans</item>
</style>

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View vista = convertView;

    // layout for spinner widget

    if (vista==null) {
        LayoutInflater inflater = actividad.getLayoutInflater();
        vista = inflater.inflate(R.layout.fila_colores_spinner, null);
    }

    return vista;
}

@Override
public View getDropDownView(int position, View convertView,ViewGroup parent) {
    View vista = convertView;

    //layout for spinner popup

    if (vista==null) {
        LayoutInflater inflater = actividad.getLayoutInflater();
        vista = inflater.inflate(R.layout.fila_colores_spinner_popup, null);
    }

    return vista;
}

ArrayAdapter<String> adapter = new ArrayAdapter<String>(
      MyActivity.this,
      R.layout.custom_spinner_item, strings);

<Spinner
      android:id="@+id/myspinner"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:background="@drawable/list_selector"
      android:drawSelectorOnTop="true" />