Function material_band 

pub fn material_band(
    theme: &Theme,
    ground: AlphaColor<Srgb>,
) -> (AlphaColor<Srgb>, AlphaColor<Srgb>)

The darkest and the lightest colour the floating material shows over ground: the ground stepped toward each end of the theme’s neutral ramp until it stands MATERIAL_SPREAD off it (or reaches the end).