QTabBar and QTabWidget Scrollbars don't have the right colours in Dark mode on macOS Mojave

If you build and run the attached test application in Dark mode, everything looks fine when the application is active (test-active), except for the unselected tab in the QTabWidget object, which is semi-transparent while I would expect it to be opaque. Still, this is roughly in agreement with Apple's Grapher application (grapher-active).

Now, if the test application is inactive then you can't distinguish the selected tabs from the unselected ones at all (test-inactive). Also, as before, the tabs are still semi-transparent in the QTabWidget object. In Apple's Grapher application (grapher-inactive), we can see that the label of selected tab gets darker and its background lighter, which is also what I would expect using QTabBar and QTabWidget.