You can keep your site visitors on the default theme while testing new themes in your WordPress site.
Theme Test Drive is a WordPress plugin that simply make your site visitors to use default theme at the time you are testing other themes. Everything happens privately and they don’t even notice that you are testing a different theme on your website. The plugin also allows you to set new options to theme that you are testing.
To start testing themes, navigate to Plugins -> Add New page to find and install Theme Test Drive plugin on your website. Upon activation, simply use the ‘Options’ page to set the theme you want to test drive. The WordPress theme you select for test driving becomes visible only to site administrators.