I have tried compiling this in Distrobox and exporting it to ~/.local/bin however it still doesn’t show up in System Settings so I guess System Settings doesn’t look there. Any thoughts on the matter?
There’s no plan to support compiled C++ plugin-based theming in KDE Linux, which includes QStyles and KDecoration themes such as Klassy and Kvantum.
Instead, the plan is to immediately adopt the in-develop Union theming system as soon as it’s ready. This system supports non-C++ themes that consist entirely of CSS and images, which makes them much easier to distribute and support on an immutable system — not to mention safer to use.
You might be able to get Klassy and Kvantum et al. working anyway, but this will not be supported. It’s “going on a fun adventure/there be dragons” territory.