meanwhile, I've discovered that if you put playing tracks in the main panel, it keeps its skin settings at first, but as soon as you restart mb, it takes the main panel settings instead.
that was intentional that the override colours didnt apply when docked in the main panel (if poorly implemented) but i have changed it so the override colours are always used
And artists view is getting it's scroll bar colors from somewhere different than track detail. In Mellon remix, they turn green & purple like column browser and some sidebar panels.
i have changed the default to come from the Panel rather than ChildPanel so it should work as you expect without adjustment. Keep in mind there are also overrides available:
Content[Artwork].ScrollBarThumb
Content[Artwork].ScrollBarBackground
Content[Artwork].ScrollBar
Content[Artwork].ScrollBar.Lowlight
for the menu pressed element, its working fine for me using your WIP skin