To reproduce:
1. Drag a RadButton from the ToolBox and drop it onto the form. 
2. Drag a TelerikMetroBlue theme from the ToolBox and drop it onto the form. 
3. Change the button's theme at design time. You will notice the strange border around the control.
Please refer to the attached gif file.

However, when you save the form and reopen it the button looks ok.