Scrollbars don't have the right colour in Dark mode on macOS Mojave

If you build and run the attached test application while in Light mode then the scrollbars look fine, as expected (1a-light). If you then switch to Dark mode, the scrollbars are of the wrong colour (1b-dark). If you switch back to Light mode, the scrollbars are light again, as expected (1c-light).

Similarly, if you start the test application in Dark mode, the scrollbars look fine, as expected (2a-dark). However, if you switch to Light mode, the scrollbars don't have the correct colour (2b-light) while they do if you switch back to Dark mode (2c-dark).