by Mike Martel on WordPress.org
Live Theme Preview allows users to preview themes on their own website before customizing or activating them.

Live Theme Preview
With Live Theme Preview, you can select a theme in an interface based on the Theme Customizer in WordPress. A sidebar shows all installed themes (or, with multisite – allowed themes), to preview, edit or activate.
See the screenshot, or install and try it for yourself.
If you want to help develop this plugin, visit the GitHub repo.
Live Theme Preview is part of Live Admin and works great with Live Dashboard, WP Getting Started and Live Editor