Assign a specific WordPress theme to any post or page.
This plugin uses WordPress's Meta Box API—supported by Gutenberg—to store per-post theme settings, which makes it 100% compatible with the new block editor.
Menus, Widgets, and More
You may want to temporarily activate (e.g., on a staging site) each theme to
properly set up menus and widgets. (When viewing a page in the Customizer, you
should see the theme options for that page's theme, but the menu and widget
locations will likely be those of the overall site theme. This probably won't be
anytime soon ever.)
Note that (using WP-CLI, for example) you can copy theme settings to, e.g., child themes. Like, you could set up your overall site, then assign a child theme to a specific page, copy over the parent theme's options (think widgets, and menus), and then start customizing (like, assigning a different layout or color scheme to said page). No need to temporarily mess up your entire site.
Custom Post Types
As WordPress sets up the active theme (the thing we are overriding) before most plugins define their Custom Post Types, there is no out-of-the-box support for them.