You are right that you need to edit the default Style of the header of RadTileViewItem in order to achieve your requirement. You can find more information about extracting styles in this article
. If this is not convenient you can open the Telerik.Windows.Controls.Navigation.xaml
file placed under you installation folder followed by Themes.Implicit\WPF40\Windows8\Themes
. In that file you will be able to find the style that you look for.
Once extracted the button that you need to replace is called "MaximizeToggleButton". Please try out this approach and let us know if you need any further assistance.
Pavel R. Pavlov
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