message: fix link hover
diff: import PropTypes from 'prop-types'
  import styled from 'react-emotion'
  import {withSystemProps, TYPOGRAPHY} from './system-props'
  -import {colors} from './theme'
  -import {themeGet} from 'styled-system'
  +import theme from './theme'
  +import {style} from 'styled-system'
  -const getHoverColor = (hoverColor = 'blue.5') => {
  - const path = `colors.${hoverColor}`
  - const fallback = colors.blue[5]
  - return themeGet(path, fallback)
  -}
  -
  -const hoverStyles = ({...rest}) => {
  - return {
  - color: getHoverColor(rest.hoverColor)(rest)
  - }
  -}
  +const hoverColor = style({
  + prop: 'hoverColor',
  + cssProperty: 'color',
  + key: 'colors'
  +})
  const styledLink = styled('a')`
  text-decoration: ${props => (props.underline ? 'underline' : 'none')};
  &:hover {
  text-decoration: underline;
  - ${hoverStyles};
  + ${hoverColor};
  }
  `
  +styledLink.defaultProps = {
  + theme
  +}
  +
  styledLink.propTypes = {
  href: PropTypes.string,
  underline: PropTypes.bool
  