I am afraid it is not possible to use the Theme through XML, our theming mechanism does not support that approach. I suggest you use the conventional way of saving user's choice of theme - using MVVM pattern, you could have a property in your ViewModel that will be of type Theme and will be responsible for the current theme. It will also update all controls when the theme changes, if you set the telerik:StyleManager.Theme property in each control like this:
To change the appearance of the day button, you should modify the DayButtonStyle. You can get that style from Expression Blend by clicking Edit Additonal Template > Edit DayButtonStyle. In the control template of the style, add your own visual element and make it visible whenever your scenario requires it. The DayButtonStyle is applied to all buttons in the calendar view, so in case you wish to apply it to a particular type of days only, you will also need a DayButtonStyleSelector to distinguish between the days and their visual representation.
I hope this information will be helpful.
All the best,
the Telerik team
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