gui: fix dark mode change on Windows 10 with universal style

gui: fix dark mode change on Windows 10 with universal style

The _Universal_ QML style has a theme that uses the colour scheme from
the system.  This can be set by either configuring
`qtquickcontrols2.conf` or setting the environment variable
`QT_QUICK_CONTROLS_UNIVERSAL_THEME=System`.

Unfortunately, due to [QTBUG-128825], setting the theme in the
configuration file/environment variable results in the dark/light mode
setting only be set during startup, not during runtime.  Not ideal, but
still better than not having a broken dark mode theme at all.

Fixes #7991

[QTBUG-128825]: https://bugreports.qt.io/browse/QTBUG-128825

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>