Page Themes

Assign a specific WordPress theme to any post or page.

Gutenberg

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 fixed 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.