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

[Jürgen Spitzmüller]

Am Montag, dem 21.10.2024 um 10:36 +0000 schrieb Juergen Spitzmueller:
> commit 4560cfd0126e11919a20a671944cad97363d8ba9
> Author: Juergen Spitzmueller <spitz at lyx.org>
> Date:   Mon Oct 21 12:34:57 2024 +0200
> 
>     Fix dark/light mode setting and run-time switch with Qt 6.8
>     
>     The old method doesn't work any longer. This one is also
> announced to
>     work cross OSes -- verification on Mac and Win is needed!

Stephan, Eugene, could you test whether this works with Qt 6.8 and
Mac/Win?

If so, this is candidate for stable.

-- 
Jürgen