[LyX/master] Fix dark/light mode setting and run-time switch with Qt 6.8

[Jürgen Spitzmüller]

Am Montag, dem 21.10.2024 um 19:12 +0200 schrieb Stephan Witt:
> In general Qt6.8 is announced to support macOS 12, 13, 14, 15. So we
> will loose many elder target platforms we currently support.

I'm not proposing that we generally switch to 6.8., I just want to make
sure it works on Mac, and there are no drawbacks.

Enjoy your holidays!

-- 
Jürgen