rounded container style

There is a much simper solution.
You don't need anything but Border:

<border borderthickness="1.2 1.2 0 0" cornerradius="3.2" borderbrush="Black" removed="Yellow" height="20" margin="6">
  <border borderthickness="0 0 1.2 1.2" cornerradius="3.2" borderbrush="White">
    <combobox>
      <comboboxitem>
         <textblock removed="Red">Red</textblock>
      </comboboxitem>
      <comboboxitem>
         <textblock removed="Yellow">Yellow</textblock>
      </comboboxitem>
      <comboboxitem>
         <textblock removed="AliceBlue">Alice Blue</textblock>
      </comboboxitem>
    </combobox>
  </border>
</border>

This code sample demonstrates more advanced style: recessed or elevated rounded border around some element.

—SA

This sample shows how to make

thx guys i now this method but i want to do it with style , to re-use it in many controls, the second idea is that i want this border to a container like grid or canvas or dock panel or any other container control.

thx for you help again