A block theme, or block-based theme, is the same as a full site editing (FSE) theme.
A classic theme is one that doesn’t work with full site editing.
Terminology
Whether you are a site owner or a developer, there’s new lingo to learn. Get started with our Full Site Editing glossary.
Differences in User Experience
If you use a classic theme, you will still be using the block-based editor for your posts and pages, but you’ll be dependent on your theme to provide editing capabilities for other parts of your site, like the header, footer etc.
Classic themes utilize either their own custom options panel, or the WordPress customizer to provide design and other controls.
They utilize “widgets” as a way to allow users to add pieces of content to sidebars and other parts of the site the theme has designated as widget-enabled.
Block themes do everything through the new Site Editor. You’ll be able to edit more parts of your site using a native WordPress interface. This will provide a more consistent experience no matter which theme you are using.
You will need to get used to the new user interface, and find where the different options are, but after that initial “culture shock” it should be a better experience.
The Site Editor
This interface allows you access to edit parts of your site using blocks much more easily.
Navigation
Navigation menus now have their own block, and are managed from within the Site Editor:
Templates
Templates control the design of specific types of pages on your site. For example, your category archives have a template, your blog homepage has a template, single posts have a template etc.
You can also create various layouts that you can choose to apply to any page, for example, a layout with a sidebar, one without a sidebar etc.
In classic themes, unless your theme gave you control, you had to use PHP to do custom layouts templates.
Now templates are available for editing in the Site Editor, using block tools and preset patterns.
As a user, block themes will give you a lot more control over your site, without needing to tinker with code.
Global Styles
These are preset color palettes and typography styles provided by your theme, that you can select and edit to style your site. This will allow you to create an aesthetically pleasing site even if you aren’t a designer. In addition to color palettes and typography, you can also set global styles for specific blocks, set layout widths and more.
Patterns
These are pre-styled and pre-configured groups of blocks that you can use to make your pages look professionally designed. Patterns are shortcuts to getting a particular look or feature on your page.
They can save you time and make your site look better than if you did everything yourself (assuming you are not a professional designer).
Patterns can be provided by both plugins and your theme.
For example, the Ollie theme provides many patterns for elements such as Calls to Action, and even full page layouts.
Once added to a page you can then edit the content as normal.
Font library
The font library feature is by default only available in block themes. It allows you to manage fonts from the WordPress site editor, making it easier for users to control the typography of their site. Themes can install fonts as part of their design. And users can upload their own, or install them from Google Fonts, and serve them from your own domain.
With some code, the font library can be added to classic themes.
Consistency
Although some classic themes give you the same amount of control, the methods and interface will vary form theme to theme. So if you change theme you have to re-learn how to control it.
With these controls built into a native WordPress system, the experience will be more unified so you won’t have a learning curve for every theme.
Technical differences
Under the hood a block theme and a classic theme are quite different.
As a developer, the transition might be more difficult because the underlying technology is different and you’ll have to re-learn how to make a theme.
Classic themes mostly utilize PHP, whereas block themes rely more on HTML, and WordPress’ new block markup.
Whereas functions.php was previously the powerhouse of your theme, themes.json is now doing the heavy lifting.
In some ways this could make theme development a bit more accessible.
On the other hand, creating custom blocks is more complicated and requires JavaScript knowledge. You can create a theme without custom blocks, of course, but taking that next step will be a hurdle.
There are some ways in which the classic theme methodologies are still supported in block themes. For example, if the block theme does not have the relevant template html part, it can fallback to a php file. But that PHP file has to be written in a specific way.
Easier theme switching
Some classic themes relied on elaborate shortcodes to create layouts. That meant if you changed themes, you were left with unrendered shortcodes in your pages.
With everything in blocks, even things like patterns which may have come with your theme, will still render to some extent. They may lose some styling, but your content will still be there in a native WordPress way.
What are hybrid themes?
Hybrid themes are classic themes which have some elements of block themes added to them. They provide a bridge between the two kinds of technologies, in theory giving you the best of both worlds if you’re not ready to make the jump to block themes.
Examples of block themes
If you are looking to experiment with full site editing and block theme, I recommend you set up a test site using Local WP or a similar tool, and get started. Here’s a few free themes you can install to play with:
Check out https://fullsiteediting.com/themes/ for a more comprehensive list.