Thanks for the reply.
Actually we just want to give rounded corner to the title of the group which contains the SubGraphExpanderButton & the TextBlock. I tried your suggestion but that makes the whole group to be enclosed inside the RoundedRectangle, which we do not want.
If you notice in the above picture, SubGraphExpanderButton & TextBlock are enclosed in a dark orange rectangle. We just want rounded corners for this rectangle. I am following SubGraphExpanderButton design - #8 by walter to change the design of SubGraphExpanderButton. Thanks