ArchLinux is a rolling release distro depending on the most recent upstream packages (usually installed `as is' with no modifications). XFCE 4.14, especially: xfce4-settings 4.14.0-1 After 4.12 to 4.14 upgrade in Appearance -> Style I can see a lot of themes I can choose with no effect including a lot of `Xfce-*' like: `Xfce-4.0', `Xfce-4.2', ..., `Xfce-4.6', `Xfce-b5', `Xfce-basic', `Xfce-cadmium', ... If those are unsupported by the most recent XFCE (or GTK it depends on) hiding invalid ones them would be a smart idea..?
I think you still have the package "gtk-xfce-engine" installed, which is obsolete and not supported anymore since release 4.14. You should remove this package.
1. If it is not supported anymore I think I'd be smart to ignore it in settings. The problem is I can see some themes and choose it even if they are unsupported. Such fake settings are confusing. 2. I've uninstalled it. Many have been removed but I still can see some themes which seem to have no effect when chosen: Adwaita, Crux, Industrial, Mist, Raleigh, Redmond, ThinIce etc.
GTK2-only themes are not invalid, but they could be marked in the Appearance Settings window. Similar to how icon themes without cache have a warning symbol plus explanatory tooltip.
Simon Steinbeiss referenced this bugreport in commit e8dd374c25fd853ec67094837ad7d503132aeb42 appearance: Only show themes that support Gtk3 (Bug #15849) https://git.xfce.org/xfce/xfce4-settings/commit?id=e8dd374c25fd853ec67094837ad7d503132aeb42
Simon Steinbeiss referenced this bugreport in commit 685dcbddee528cc8e4f6736e94a072ea33441625 appearance: Only show themes that support Gtk3 (Bug #15849) https://git.xfce.org/xfce/xfce4-settings/commit?id=685dcbddee528cc8e4f6736e94a072ea33441625
From xfce4-settings 4.14.2 onwards we will now hide themes that don't support Gtk3.