Actually the mentioned forum thread is quite old. However, if you want to change these colors, you need to predefine the template of GridViewRow element. Then you can modify the Selected/MouseOver visual states.
I would suggest to work with our Implicit Styles as demonstrated in this help article
. For your convenience, I have prepared a sample project showing you the approach. Please check the changes I made in the Themes/ Telerik.Windows.Controls.GridView.xaml file.
I hope it helps.
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