This code is used like that for years in KDevelop and works quite
    nicely for most cases.
    
    Note that for darker color schemes, this most often results in
    a bit less-intense colors which is not a big problem as colors stand
    out much more against a dark background. And its better than before,
    where you'd end up with hard-coded colors for a bright background
    scheme...
    
    Bright color schemes are not affected, as long as they fulfill the
    conditition of having a lower luma for the background color compared
    to the foreground color.

Created a new kate scheme with a dark and with a bright system color scheme. The bright one stayed as-is. The dark one auto-adapted all colors - even those hardcoded in highlighting .xml files!

The result is an always-readable text on dark schemes, where previously hard coded colors lead to unreadable mess which one had to fix manually...