You are most probably using the Aero Theme; the Aero look does not use the "SelectionBackground" property, this is why your code did not have an effect. For the Aero theme, to change this, you will need to make a copy of the ControlTemplate for the DataRow (provided with the package) and modify it to your needs.
Or if its possible, just change the theme and your code will work:
Xceed Technical Support
Xceed Software Inc