WPF and Win32 are very different technologies, which is why it is so easy to port WPF to new operating systems: It has practically no reliance on Win32.
When it comes to custom themes, WPF templates completely changes the ballgame: Custom themes in WPF are much more powerful but also very different from Win32. This means that theme vendors must implement their theme for both Win32 and WPF.
If system theme includes both a Win32 and a WPF DLL, all you need to do is make sure the WPF DLL is in your application directory or in the GAC. WPF will automatically load it and use it based on the registered theme name. Installing a theme on the system that includes a WPF DLL should automatically register it in the GAC.
If the theme includes only Win32 code and doesn't include any WPF templates, the only way to get the same look and feel from WPF is to code your own templates to match those in the theme. Fortunately this is very easy to do.