To reproduce:

1. Add a RadForm and a RadTimePicker/RadDateTimePicker.
2. Change the Enabled property of the RadTimePicker/RadDateTimePicker to false at design time.
3. Apply the Office2013Light theme to the RadTimePicker/RadDateTimePicker at design time.
4. Apply the same theme to the RadForm at design time.

If you run the application, you will notice that RadTimePicker/RadDateTimePicker has a gray back color. However, if you turn on/off the Enabled property in a button's Click event, the back color is not gray.

Workaround: set the Enabled property to false in the Load event.