How can I set theme per-application?

Hi, is there a way to set xfwm theme for a specific application?

I can set GTK theme with: g_object_set (gtk_settings_get_default (), "gtk-theme-name", new_theme, NULL); How can I set XFWM theme?

Why? This is for a theme preview application, when I set a theme, the window theme is not updated, and this is not good.