The dark mode is only turned on if Windows 10's own dark mode is
enabled. This manual dark mode configuration is necessary as Qt itself
doesn't support Windows' dark mode yet; see the following bug:
https://bugreports.qt.io/browse/QTBUG-72028
This isn't a problem for other operating systems (Linux, macOS…) as Qt
uses the system colors there and will use dark colors when the
system's colors are set to dark colors, too.
