Usually in Metro theme the color for the read only and normal DateTimePicker is the same. Nevertheless we have attached a simple project showing how you can change the background in read only state and use the implicit style
in order to apply the theme and therefore the change to all of your DateTimePickers.
Please note that this is applied according to our latest version (Q3 2012 SP1) and there might be some differences if you are using different control version.
You can find the changes in Telerik.Windows.Controls.Input.xaml as we have added the following code in the RadWatermarkTextBoxStyle:
Hope this will help.
the Telerik team