WPF - ListBox Background Color - CodeBehind

I am trying to set the background color for a list box in code. I can get it to work with the list item, but not the list itself.

Here's the code that works (with a ListBoxItem):

        private void SetBackgroundGradient()
    {
        var styleListBox = new Style(typeof(ListBoxItem));

        var myBrush = new LinearGradientBrush();
        myBrush.GradientStops.Add(new GradientStop(Color.FromArgb(255, 0, 0, 0), 0.0));
        myBrush.GradientStops.Add(new GradientStop(Color.FromArgb(255, 255, 255, 255), 1.0));


        styleListBox.Setters.Add(new Setter
        {
            Property = BackgroundProperty,
            Value = myBrush
        });

        lstTopics.ItemContainerStyle = styleListBox;
    }

      

        
        
        
      

    

Now if I change the code to try and work with the ListBox itself, all I get is a white background. Here's the code for that:

private void SetBackgroundGradient()
    {
        var styleListBox = new Style(typeof(ListBox));

        var myBrush = new LinearGradientBrush();
        myBrush.GradientStops.Add(new GradientStop(Color.FromArgb(255, 0, 0, 0), 0.0));
        myBrush.GradientStops.Add(new GradientStop(Color.FromArgb(255, 255, 255, 255), 1.0));


        styleListBox.Setters.Add(new Setter
        {
            Property = BackgroundProperty,
            Value = myBrush
        });

        lstTopics.Style = styleListBox;
    }

      

        
        
        
      

    

Any idea what I might be doing wrong?

If you need to clarify what I am asking for, please let me know.

Thanks in advance.