A new experimental package joins the BeeWare yak stable: system-pyside6
If you're on a Linux machine with a KDE desktop, you already have Qt installed, with a system theme. But a virtual environment won't inherit this theme, as the PySide6 packages install a full duplicate copy of Qt.
system-pyside6 is an importlib hack that exposes the system-provided PySide6 libraries into a virtual environment.