Klassy is a highly customizable binary Window Decoration, Application Style and Global Theme for KDE Plasma.

In general it is *faster and snappier* than other 'normal' window decoration / aurorae (because its a library, installable systemwide).

after installing the deb (qt6.8+ / plasma 6.32+) make sure
* it is enabled in System Settings -> Appearance -> Window Decorations,
* in System Settings -> Appearance -> Application Style and
* in System Settings -> Appearance -> Icons.