Function selected_edge 

pub fn selected_edge(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    radius: f64,
    theme: &Theme,
)

The selected object’s edge, in place of the rest edge card paints: at the same position, a heavier line in the accent drawn wholly inside the surface’s outline. One outline, with no gap and no ring outside it. Every selectable card or tile marks its selected state with it.