How to change the prefix background of the RibbonMenuButton items?

Question

I write a sample ribbon here and tried to changed the background of the item as indicated by the #2 arrow in this picture:

I tried to use all the properties related to the background but none can do that. Please help!

[Edit]

Here are some links I found that would be helpful to get started:

1. Control Styles and Templates (look under the ListView and ListBox section)