The Preview Theme Plugin allows you to take a quick peek at a theme without having to switch to it via the Presentation manager. 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 any 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 plugin. The required admin level and the name of the query argument can be changed. Here’a an example using the default settings.
Note that if you want to switch to a theme and have a cookie follow you around, you could use the Theme Switcher for your previewing needs. However, the theme switcher does not perform user level checks. Readers will be able to peek at your in-progress themes whenever the switcher plugin is active.