3.0 Demo: Styles without all three components

Example style: http://www.phpbb.com/community/viewtopic.php?f=73&t=1157015

We allow submissions for 3.0 where only one or two of the three components can be submitted (imageset, theme, or templates). In the above example, only the theme and imageset were included, since it uses prosilver templates. This throws an error upon approval and automatic installation to the demo.

It is important that we are able to install such styles to the demo, and any "missing" components just be read from the default prosilver.