the files for you theme aren't in the database so you can't use phpmyadmin to do this.
if you've got a back up of your site then it should have a directory structure with a folder called wp-content.
inside this is a folder called themes, find you theme in there and upload the missing files from your back up. don't touch the database unless you really know what you're up to as it could break your site in one of thousands of ways!