You might have seen the awesome Klassy theme by Paul McAuley for Qt applications and window decorations for KWin.

Klassy
Klassy

It has some issues compiling against the latest Plasma since the KDecoration API break.

Until it is fixed in the main repository, I’ve created a temporary fork that includes the port to KDecoration3 done by Eliza Mason, with a tiny additional fix I added on top of it. The fork is available at github.com/ivan-cukic/wip-klassy

The kdesrc-build recipe for it is:

module klassy
    repository https://github.com/ivan-cukic/wip-klassy
    cmake-options \
        -DBUILD_QT5=OFF \
        -DBUILD_QT6=ON
    branch plasma6.3
end module