I have read the suggested topic answer at Multiple Child Themes but I have further questions.
I am planning a site with different sections that will each have a different look and feel. I expect the theme changing plugin “Multiple Themes” https://wordpress.org/plugins/jonradio-multiple-themes/ to be able to handle that based on url’s as, from what I’ve read, it sounds fairly robust.
I am more concerned about the plugins and widgets. Can you foresee problems with these across multiple themes?
If it is going to create big problems, I’m considering another option - a different WP installation for each of these sections.