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

[Jürgen Spitzmüller]

Am Dienstag, dem 22.10.2024 um 17:12 +0200 schrieb Yu Jin:
> A runtime switch? I don't see any switch, The only switch I get is
> when using #12224 patch.

On Linux and Mac, when switching the OS between dark and light, an
opened LyX instance inherits this switch automatically. AFAIK this
didn't work on Windows up to now since there was no reliable signal.
According to the Qt docs, the ThemeChange signal now is emitted on all
OSes, hence I figured this could work now on Windows as well.

-- 
Jürgen