I want to leave that behavior as a default but i am willing to add a boolean override to MB just uses the exact colours as specified in your skin to the user-set narrow scrollbar
After looking through a lot (lot = all

) of skins with narrow scrollbars enabled I've concluded that if you have a skin with strong contrast between the panel bg colour and the scrollbar slider colour (eg. MusicBee3, Tron Punk), then everything works fine. If the colours are closer together (hue-wise) than the narrow scrollbars dont work too well (eg. Arsenic, DarkRED).
Rather than the boolean override you have suggested, a new skin element
<element id="Panel.ScrollBarNarrow.Highlight" bg="XXX" bdr="XXX"/>
that is only used if present, otherwise MB reverts to using
<element id="Panel.ScrollBar.Default" bg="XXX" bg2="XXX" />
as is currently implemented.
I know this would also entail adding;
id="Controls.ScrollBarNarrow.Highlight"
id="FilterPanel.ScrollBarNarrow.Highlight"
etc, etc
but it would not only solve my problem, it would allow the use of a contrast border on the narrow scrollbar slider when a skin is still using scrollbar thumb arrows.
Is this solution a possibility?