If you applied some of the predefined skins for the RadCalendar (for example the Default skin), you can easily model its look and feel by the following style properties:

• CalendarTableStyle
• DayOverStyle
• DayStyle
• DisabledDayStyle
• OtherMonthDayStyle
• OutOfRangeDayStyle
• SelectedDayStyle
• HeaderStyle
• TitleStyle
• WeekendDayStyle
• FastNavigationStyle

You can read more about RadCalendarStyle by the following help article:
Appearance Style