I’ve changed themes, activated each one, but they just won’t apply. The initial theme remains the same. I cleared the cache and cookies but still the same. I tried in different web browsers, both in Chrome and Firefox, but still the same.
I did take a quick look at your site and I see you have the Pagelayer page builder installed. I’m not very familiar with that builder, but plugins that completely revamp how theming works in WordPress may also interfere with the regular theming selection. So maybe there are some settings in Pagelayer which you need to tweak as well.