to decorate your assistant pages, gtkassistant provides similar functions to gnomedruid, so you have to transform code like this:

gnome_druid_page_edge_set_title (gnome_druid_page_edge (page), "welcome");
gnome_druid_page_edge_set_logo (gnome_druid_page_edge (page), logo_pixbuf);
gnome_druid_page_edge_set_watermark (gnome_druid_page_edge (page), watermark_pixbuf);
    

into this:

gtk_assistant_set_page_title (gtk_assistant (assistant), page_widget, "welcome");
gtk_assistant_set_page_header_image (gtk_assistant (assistant), page_widget, logo_pixbuf);
gtk_assistant_set_page_side_image (gtk_assistant (assistant), page_widget, watermark_pixbuf);
    

where page_widget is the widget used as a page.