Other things which lie around a file
The most important other thing in a theme file is the metadata held in the info tag. This contains a set of tags each of which contains some text explaining something about the theme itself, in a sort of Dublin Core sort of way. (Next time around, we should probably use the actual Dublin Core.) The tags are name, author, copyright, date, and description.
Version 1 of the format had a menu_icon tag at top level, which let themes specify the icons beside options in the menu you get from the menu icon. This has become redundant; the icons are taken from the icon theme! The tag can still be used in all formats, but does nothing and is deprecated.
Version 2 of the format has a fallback tag at top level, which let the theme specify what icon a window should be considered to have if it doesn't provide an icon of its own. This should also be taken from the icon theme, if anyone fancies fixing it, and the tag should also then be deprecated. It shouldn't be hard.