Inherit theme setting from parent group when using og_subgroups

The og_subgroups module allows have a heirarchy of groups where a group can contain other groups. It'd be great if child groups inherited their parents theme if the setting was left at the default!

I've created a patch (which I'll upload after I have the issue NID), which:

1. Will check the parent groups if the current group doesn't have a value set.
2. Changes the default option from 'Use site-wide theme definition' to 'Use default theme' because the default might be the parent's setting rather than the site-wide default.