https://drafts.csswg.org/css-scrollbars-1/#scrollbar-color WebKit should implement the "scrollbar-color" property. This is a standard alternative to using the various "::-webkit-scrollbar" Pseudo elements and their background-color properties.
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<rdar://problem/84418022>
For anyone following this issue there's currently a limitation with the platform APIs that blocks this property being implemented on macOS and iOS.