function shade(percentage, color) {
    if (color === 'transparent') return color;
    return mix$1(parseFloat(percentage), 'rgb(0, 0, 0)', color);
  }
Shades a color by mixing it with black. `shade` can produce
hue shifts, where as `darken` manipulates the luminance channel and therefore
doesn't produce hue shifts.

@example
// Styles as object usage
const styles = {
  background: shade(0.25, '#00f')
}

// styled-components usage
const div = styled.div`
  background: ${shade(0.25, '#00f')};
`

// CSS in JS Output

element {
  background: "#00003f";
}