[LyX/master] Change preedit color settings

[jspitzm at gmail.com]

Am Montag, dem 06.07.2026 um 12:36 +0000 schrieb 横田　宏治:
> Oh, thanks a lot! I was searching what was wrong :-)

Basically the problem was that the method that checks whether a color
is "light" or "dark" failed in the case of transparent colors, since
the transparent color is defined as #00ffffff (transparent white) which
was returned to that method as "white" RGB, which is "light", whereas
the background is actually dark in dark mode (when overlayed to a dark
background).

I have now changed to RGBA for all these methods, and in the check, the
colors are overlayed with their background.

A simpler fix would have been to defined a dark_transparent color
(transparent black) and use that for dark mode, but this would still
have failed for user-defined transparent colors.

Jürgen