Function thumb_color 

pub fn thumb_color(theme: &Theme, active: bool) -> AlphaColor<Srgb>

The thumb’s fill: the secondary ink while hovered, dragged or focused, stepped out until it clears the non-text floor (3:1) against the ground it scrolls over. At rest it is the quiet tertiary ink on a dark ground; on a light ground the same ink reads as a heavy dark bar, so it rests a third of the way back toward the ground (it still clears the floor once the pointer is on it). One answer for every scroll owner.