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

[Jürgen Spitzmüller]

Am Dienstag, dem 22.10.2024 um 19:11 +0200 schrieb Yu Jin:
> Well not forever, but for pretty long, like 20-30 seconds. And after
> that the ui is broken, partially in dark and partially in light color
> scheme (tried windows11 (default) and fusion styles).

Which parts are dark and which are light? Or more precise: which are
not correct? Maybe we can fix that. Otherwise I'm just going to
deactivate this feature on Windows.

-- 
Jürgen