Qt 6.4.0 has added dark theme support on Windows 11 with the updated built-in style Fusion, which seems to be a perfect replacement for the custom psblack style sheet.
We can upgrade the project to Qt 6.4.0, and we'll be able to use the updated dark Fusion theme.
Or, alternatively, we can manually migrate the feature from Qt source code found here.
Below is how Fusion style looks like on my own app with Qt 6.4.0:
