Multiple Themes with Custom Permalinks

I’m trying to set up multiple themes for categories.

The WordPress site is using custom permalinks. I’ve figured out how to create and use multiple page themes on a site, but can figure out how to apply multiple themes to categories when the custom permalink isn’t a default numbered category generated by WP, but a named categoy in a custom permalink. OK, I’m going to guess the answer is something kind of obvious, but really appreciate someone pointing me in the right direction.