So libadwaita is basically a library that provides a set of user interface elements and patterns for building GNOME applications. It uses GTK4, but the main difference is that it has the specific Adwaita theme "baked in", and that you "can not change it".
Of course, you can change it. The gnome folks just made it harder than normal, by not making it follow the established theme folder configuration. But in the end, all you have to do is create a symlink. That's basically what this tool does for you: https://github.com/odziom91/libadwaita-theme-changer
But now I found another solution, which does make libadwaita follow the selected GTK4 theme, and it's called "libadwaita-without-adwaita"
Great stuff! One less hoop to jump through when I want to enable the Skeuos theme 😄