A hybrid theme is a classic theme that has adopted some block theme functionality. This is not an official term, but every professional WordPress developer will understand you if you call a theme “hybrid”.
Hybrid themes exist for a couple of reasons. Some classic themes predating the block system are so complex and widely used it’d be impossible to convert them to block themes. Yet, the block-based APIs have been evolving over the past few years, and it’s very clear blocks are not going away any time soon.
It’s in some limited ways possible for classic themes to utilize block theme functionality without becoming a block theme. Let me remind you – a block theme is a theme with a /templates/index.html file present.
There’s no single set of things a classic theme has to do or support in order to be classified as a hybrid theme. The most relevant things to look for are either add_theme_support() calls enabling block-based functionalities or the presence of a theme.json file, with emphasis on the latter. Yes, a classic theme can have a theme.json file.
You can use the theme.json to configure settings and styles, just like you can in block themes. This is, in it of itself, very powerful. You can provide color palettes and default styling to blocks, which is invaluable even in classic themes, as the Block Editor is now the default editor for every post and page.
Keep in mind though, the Site Editor experience will be very limited. I have not found a way to enable style editing (only previewing) or to even get the navigation, pages, or templates tabs to show up. The latter is to be expected as hybrid themes are not meant to support FSE – they are still classic themes after all.
You can also create HTML template parts and use them in your PHP templates. You can render a block-based part in PHP using the block_template_part() function. You can even edit template parts and add patterns in the patterns tab of the Site Editor, just like you could in block themes.
If you add HTML templates to your theme’s /templates directory, they’ll be used instead of their PHP counterparts. Yes, block templates can be used and are prioritized even if the theme is not a block theme. But as already noted, you can forget about editing them from the Site Editor.
PS: The Site Editor in hybrid themes will most likely be named “Design” instead of “Editor” in the Appearance menu.