To achieve this you have to edit the template of the RadNumericBox. As the default template relies on predefined Telerik named resources(e.g. TelerikNumericBoxWatermarkForegroundBrush), you have to add additional code into your App.xaml file. For more information you can see this article from our online documentation: Resolving Telerik named resources
I have attached a runnable sample application that demonstrates how to change the RadNumericBox template. If you wish to apply additional styling, please uncomment and edit the relevant setters in the custom style.
I hope this helps. Please, do not hesitate to contact us if you have further questions.