NavItem.js

import { Link } from 'react-router';
import styled from 'styled-components';

const NavItem = styled(Link)`
  background-color: blue;

  &.${(props) => props.activeClassName} {
    background-color: paleturquoise;
  }
`;

NavItem.defaultProps = {
  activeClassName: 'active',
};

export default NavItem;