These properties are not used. Sorry for the inconvenience caused.
In order to customize the content of the Increase/Decrease buttons you will need to customize the ControlTemplate that targets RepeatButton. In order to customize the ControlTemplate of the two buttons you will need to copy the ControlTemplate that targets telerik:RadNumericUpDown from the Telerik.Windows.Controls.Input.xaml file in the Themes.Implicit folder in the installation folder and then customize the two separate ControlTemplates.
Hope this helps.
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