Title: start_previewing_theme
Published: April 25, 2014
Last modified: April 28, 2025

---

# do_action( ‘start_previewing_theme’, WP_Customize_Manager $manager )

## In this article

 * [Parameters](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#parameters)
 * [Source](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#source)
 * [Related](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#related)
 * [Changelog](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#changelog)

[ Back to top](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#wp--skip-link--target)

Fires once the Customizer theme preview has started.

## 󠀁[Parameters](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#parameters)󠁿

 `$manager`[WP_Customize_Manager](https://developer.wordpress.org/reference/classes/wp_customize_manager/)

[WP_Customize_Manager](https://developer.wordpress.org/reference/classes/wp_customize_manager/)
instance.

## 󠀁[Source](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#source)󠁿

    ```php
    do_action( 'start_previewing_theme', $this );
    ```

[View all references](https://developer.wordpress.org/reference/files/wp-includes/class-wp-customize-manager.php/)
[View on Trac](https://core.trac.wordpress.org/browser/tags/6.9.4/src/wp-includes/class-wp-customize-manager.php#L708)
[View on GitHub](https://github.com/WordPress/wordpress-develop/blob/6.9.4/src/wp-includes/class-wp-customize-manager.php#L708-L708)

## 󠀁[Related](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#related)󠁿

| Used by | Description | 
| [WP_Customize_Manager::start_previewing_theme()](https://developer.wordpress.org/reference/classes/wp_customize_manager/start_previewing_theme/)`wp-includes/class-wp-customize-manager.php` |

If the theme to be previewed isn’t the active theme, add filter callbacks to swap it out at runtime.

  |

## 󠀁[Changelog](https://developer.wordpress.org/reference/hooks/start_previewing_theme/?output_format=md#changelog)󠁿

| Version | Description | 
| [3.4.0](https://developer.wordpress.org/reference/since/3.4.0/) | Introduced. |

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