Safely test drive any theme as an administrator, while visitors use the default one.
Upload the whole plugin folder to your /wp-content/plugins/ folder.
Go to the Plugins page and activate the plugin.
Use the Options page to set the theme you want to test drive.
The selected theme will be visible only to blog administrator.
Other visitors of the blog will always see the default theme.
Note: if you use WP-Cache plugin, you might need to disable it (or setup to exclude pages)