Function face_colors 

pub fn face_colors(
    theme: &Theme,
    role: ButtonRole,
    interact: ButtonInteract,
) -> (AlphaColor<Srgb>, Option<AlphaColor<Srgb>>, AlphaColor<Srgb>)

The colours of role in interact: the single source of truth for the retained Button and the immediate-mode Button::paint.

edge is None where the fill defines the shape by itself (an inverted primary face) or there is no face (a plain button); the painters stroke the focus ring, in your colour, on top when focused.