A theme is considered valid if there is a index.php script in 
the particular themes directory.

The 'printable' theme is required for the 'Printable version'
listings and borrow page links to work correctly.  It is 
not available as a user theme, and is programatically disabled
as a valid theme.

Please refer to the docs/notes/themes.txt document for more information
on the structure of themes.

Jason Pell
21st June 2002