If you are considering switching from one theme to another, the Switchtheme module is very useful to allow you to configure blocks and make other theme related updates on a live site, BEFORE making the new theme the default theme for all users.

1. Add the Switchtheme module and enable it (link to adding modules)
2. Give specific roles permission to switch themes (link to managing permissions)
3. Configure switch theme block to only appear for user/*/edit (link to managing blocks)

Once Switch theme is enabled and the block configured, specific users will have the option of switching between any enabled theme.

WARNING! Switch Theme does not always play well with CiviCRM and civicrm_theme. If you are using CiviCRM, you should only enable Switch Theme to preview changes to the Drupal side of the configuration.