Actually this is by design - it is made to be consistent with DateSelectionMode="Day", where you can see the days from the last and previous Month, they are gray and selectable.
If you want to change it you need to change the style of the calendar. More info on this here
If you want to change the style of the buttons from the calendar witch are gray you need to make changes on
which is in
Hope this helps.
All the best,
the Telerik team
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