Michael, go to your database with phpmyadmin or similar, look in your tiki_preferences table (or similar), and look for any *.css record. You'll find where in the database is stored that selection you made. Delete that record.
Tiki will use tikineat.css or tikinewt.css instead, and your tiki will be back up again.
Remember to dissallow your users to select those themes which are not ready for your tiki version.
Afaik, new Tiki version will come with a shorter set of theme included, all them working to that release, etc.