Help with Preview Theme plugin please :)

Hi guys

I’m using Ryan Borens preview theme plugin (https://boren.nu/archives/2005/03/08/preview-theme-plugin/) to show previews of themes on my site to users who are logged in.

I would like to modify the plugin so that any user who is not logged in is redirected to login.php when trying to preview a theme.

My php is super-poor, and I’m sure this must be super-easy to do.

Any help appreciated, thanks, pilks.