Thank you for writing.
Appointments cannot be customized through theme, so you should do that with code. However, the border of the selected appointment has custom painting and for the time being you cannot turn it off or change it. There is no suitable workaround that can be used for this case. However, in our development version we already added an improvement that will allow you to overcome this limitation and to change the border according to your needs. This improvement will be included in our next release – Q2 2013, which is planned for mid June.
In regards to the second question, you cannot take the inner border like independent element. It is part of the border primitive element. In this case to achieve the desired behavior you should simply set the BorderBoxStyle property of the appointment element to "SingleBorder". You can read more about the BorderPrimitive in this documentation article: http://www.telerik.com/help/winforms/tpf-primitives-borderprimitive.html
I hope this information helps.
the Telerik team