[LyX/master] Change preedit color settings

[Jürgen Spitzmüller]

Am Samstag, dem 04.07.2026 um 16:25 +0200 schrieb Scott Kostyshak:
> Starting with this commit, my cursor turns black when it enters a
> math
> inset. I am using dark mode, in case it is relevant.
> 
> Can anyone reproduce?

Yes. The reason is that math background has been set to transparent,
but the cursor inversion code relies on it being a non-transparent
color to determine whether it is dark or light.

-- 
Jürgen