The yellow mouse over color is the mouse over state of the ButtonChrome control which is part of all buttons in RadCalendar. If you wish to change the mouse over color, you will need to edit the style for the ButtonChrome control and its brushes.
If you are referring to the mouse over state of the DayButton, you can easily change that by generating the DayButtonStyle from Expression Blend. Then, find the SelectionChrome which is part of template in the the style. Generate the style for the SelectionChrome and modify itsHighlighted visual state.
The change will be reflected in the mouse over state of the DayButton.
I hope this helps.
the Telerik team
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