ScrollBar::backgroundColourId and ScrollBar::trackColourId are documented but afaict never referenced in the codebase. ideally these would be implemented in the drawScrollbar l+f method but at the bare minimum the references to those colourIds should be removed from the docs.
ScrollBar::backgroundColourId and ScrollBar::trackColourId are documented but not used in LookAndFeel_V4 which means out of the box neither has any effect. one could use the older versions of drawScrollbar but it’s not obvious that this is a possible solution.
so i guess the overall issue is hard to solve - in nearly every other component class setting colourIds works without a custom lookandfeel class (or without deriving specifically from an older l+f version). this was the first case i saw where it didn’t work, and it cost me some time in debugging to realize why. i guess i’ll just leave this here with the hope that it helps someone else, thanks.