On this site I wanted to make previews of themes available to users. I wanted to restrict the previews to registered users only as many of the themes are not available publicly.
The original plugin stated:
By default, if you are logged in as a level 8 or above user, you can specify “preview_theme=Theme Name” in the URI query string to preview an installed theme. Regular blog readers will not be affected by this setting. They will still see the theme selected in Options->Presentation or the Theme Switcher.
I have modified this slightly to return an unregistered user to the login/register page of the site.
I hope you find it useful.
Future plans for this plugin are to have settings to allow any user whether logged in or not to preview released themes.