different theme in one website

This is one of the best use cases for WordPress Multisite. There are some plugins (like this one) that do this on a per-page or per-category basis, but I’ve never used them.

Multisite has a lot of nice advantages (network-wide updates, user management, etc), along with some fun extra bugs (a lot of the recent core security updates have been for multisite-specific issues) and plugins that don’t work on multisite.

I would recommend reading “Don’t Use WordPress Multisite” from .org support guru @ipstenu first, but it sounds like your use case is a good one for multisite.

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