[LyX/master] Change preedit color settings

[Koji Yokota]

> 2026/07/05 2:07、Jürgen Spitzmüller <jspitzm at gmail.com>のメール:
> 
> 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.

It seems the default background color seems different between macOS and Linux. It looks fine on Mac. Let me check with it.

Koji