From the title of your question, I assume that you want to change the item's background, not the combobox's background when it's selected (which is invisible, since covered by the selected item).
There's a ComboBoxItemSelectedBackgroundThemeBrush resource for that: see
http://msdn.microsoft.com/en-us/windows/apps/jj709911.aspx .
So try this:
<SolidColorBrush x:Key="ComboBoxItemSelectedBackgroundThemeBrush" Color="#FF4617B4" />