The system font used for the application's user interface can now be set. Any C++ or Python plugin that is a child of the QGIS GUI application or has copied/applied the application's GUI stylesheet can inherit its styling, which is useful for GUI fixes across platforms and when using custom QGIS Qt widgets, like QgsCollapsibleGroupBox
.
The generated QGIS GUI application stylesheet for the current platform can be accessed after launching with (in PyQGIS):
qgis.utils.iface.mainWindow().styleSheet()