Thank you for contacting us.
Attached you will find a sample project in which a custom theme is applied to the DatePicker control. The theme works as expected. Please go through the code and if you still experience difficulties send us a sample running project, which we can test here locally of modify this one to fit your scenario. In that way we will be able to find and resolve the issue in the best manner.
Looking forward to your reply.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.