State of the Customizer with block themes in WordPress 5.9

With WordPress 5.9 bringing support for block themes, this results in a few changes to how the CustomizerCustomizer Tool built into WordPress core that hooks into most modern themes. You can use it to preview and modify many of your site’s appearance settings. works and how one can preview themes.

At a high level, this post covers how blockBlock Block is the abstract term used to describe units of markup that, composed together, form the content or layout of a webpage using the WordPress editor. The idea combines concepts of what in the past may have achieved with shortcodes, custom HTML, and embed discovery into a single consistent API and user experience. themes cannot be previewed due to the introduction of the Site Editor and how the Customizer itself will only appear if a non-block theme is activated or a site has a pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the Plugin Directory or can be cost-based plugin from a third-party that interacts with the Customizer.

Previewing block themes

If you activate a non-block theme like Twenty Twenty-One, open the Customizer, and click on the Change button in the Active theme section, you will see a warning message displayed for each block theme. Please see the example below:

A warning message is displayed for each block theme in the Customizer

It is displayed because the Customizer doesn’t support block themes. They can be customized in the Site Editor instead.

As a result, to customize a block-based theme, you need to activate it first. If you’re concerned about doing so, it’s recommended that you set up a test site first to explore the theme.

Also, it is not possible to preview inactive block themes because the Site Editor only works with the currently activated block theme. This means that the Live Preview button is not displayed for inactive block themes.

Once the ability to preview any block theme in the Site Editor is implemented, it will be possible to enable the Live Preview button for block themes as well.

Previewing non-block themes

Previewing non-block themes works the same way as in WordPress 5.8. Nothing has changed here.

Customizer and block themes

WordPress removes the Customizer from the Appearance menu if a block theme is enabled. Why? Block themes are customizable within the Site Editor rather than the Customizer.

However, if any callback function is registered for the customize_register action, the Customize menu item will be added. This is necessary to ensure compatibility with plugins that still require the Customizer to be present in the Appearance menu.

Because the Customizer shows up conditionally, various links have been adjusted depending on the type of active theme to ensure a smooth user experience.

For example, if a block theme is activated, the link in the Welcome banner leads to the Site editor. However, if a non-block theme is active, it leads to the Customizer.


If you need to preview or edit a block theme, activate it first. Previewing non-block themes will continue to work the same way.

WordPress will only add the Customize menu item if a non-block theme is activated or a plugin interacts with the Customizer (please see the customize_register action).

TracTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress. tickets: #54549 (warning messages for block themes in the Customizer), #54460 (change links depending on the type of active theme), #54337 (conditionally show the Customizer in the Appearance menu).

Props to @annezazu and @hellofromtonya for technical review and proofreading; props to @audrasjb for proofreading; props to @desrosj for providing general feedback on this post.

#5-9, #block-themes, #customize, #customizer, #full-site-editing