In order to customize the control template of RadWindow, you should get familiar with the required template parts that have specific name. This is required from the code-behind implementation of RadNumericUpDown and if you don’t use all of these controls with the same names, RadNumericUpDown will not have complete functionality. Note that your control will still build and you will not receive any notifications for missing parts.
Here is an image that describes the required items from the code-behind:
<TextBox x:Name="textbox" />
<RepeatButton x:Name="increase" />
<RepeatButton x:Name="decrease" />