If you go through the Xam Feature Browser samples (which are installed in the Start menu when you install NetAdvantage for WPF), there are various examples how to work with Themes. You can choose a theme which you think works best on the various operating systems you are deplying to.

There is no built-in way to dynamically change the theme to best match the current OS. This is something you would have to program utilizing the Themes which come with the control or utilizing your own custom themes.