Hello Tau Sick,
Thank you for contacting us.
Could you please share with us how do you set the theme, is it an application theme? If so, could you try setting the theme to the RibbonBar and to the DatePicker separately. Also, could you please let us know which version of the controls you are using.
Attached is a sample project with a RibbonBar and a DatePicker inside a RibbonGroup with themes set to Vista. Could you please try it, and if you still experience problems, send us a running project which we can test here locally or modify this one to fit your requirements.
Looking forward to your reply.
All the best,
the Telerik team
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