the look of apps created on vista is kewl and it would be nice to have the same aero theme as default when apps are run on other themes ... the way to do this is to specify the theme at the root. So it would look something like this.
<ResourceDictionary Source="/PresentationFramework.Aero, Version=126.96.36.199, Culture=neutral,
PublicKeyToken=31bf3856ad364e35, ProcessorArchitecture=MSIL;component/themes/aero.normalcolor.xaml" />
Bear in mind that the syntax uses a strong name URI. This is because the app fails to load the assembly from the GAC; the CLR does not support loading dlls with a partial name from the GAC.
To get the Version, Culture and PulblicKeyToken right go to \windows\assembly and check the values for the corresponding dll.
With the above syntax you can now build apps which look like the media player 11 on XP... have fun coding.