Always compute the grid lines colors as a function of the palette's windowtext color.
Don't be concerned if the windowtext color is from the system or custom.
note that that we don't permit custom changing of the windowtext color . we can only allow changing the background colors.

https://bugs.kde.org/show_bug.cgi?id=411608

tried various color combinations and using system colors and not using system colors.
appreciate other testers since I don't use dark theme