Support for theming in PHP templates has been dropped.
True theming should support theme inheritance, e.g. mytheme <- table <- default.
Currently, the Templating component does not support such inheritance. As the
only purpose of the themes so far was to style field groups with tables or
divs, and because automatic rendering of field groups/forms through the render()
method is discouraged and only recommended for rapid prototyping, themes are
dropped for now.
