Add support for macOS dark mode

[Knezzen]

Hi!

It would be awesome to add support for automatic macOS dark mode detection.

Thanks for an awesome contribution to the Hotline community!

[tjohnman]

Thank you for your kind words! I'm not sure dark mode can be done with Qt 4, though. And I think on Mac, color palettes for the UI are not supported, so I think there's no good way to do it.

Perhaps there could be a Qt 6 build that supports this.

[barracuda156]

It would be indeed nice to add that in a non-breaking way, since, as of now, we still only have Qt4 on some macOS versions.