message: Header changes to new design
diff: import Octicon, {MarkGithub} from '@githubprimer/octicons-react'
  -import Link from 'next/link'
  -import {UnderlineNav, UnderlineNavLink, Text, FlexContainer, Box} from '../../src'
  +import NextLink from 'next/link'
  +import {Text, FlexContainer, Link, Box} from '../../src'
  import {name, repository, version} from '../../package.json'
  const pkg = `${name}@${version}`
  const releaseURL = `https://github.com/${repository}/releases/v${version}`
  const Header = () =>
  - <UnderlineNav
  - actions={
  + <FlexContainer mr={1} p={2} alignItems="center" justifyContent="space-between" bg="black" color="white">
  + <FlexContainer alignItems="center">
  + <Octicon size="medium" icon={MarkGithub} className="mr-2" />
  + <Text>Primer-react</Text>
  + </FlexContainer>
  <div>
  - <Link href="/docs">
  - <UnderlineNavLink px={4} className="d-inline-block">
  + <NextLink href="/docs">
  + <Link color="white" px={4} className="d-inline-block text-white">
  Docs
  - </UnderlineNavLink>
  </Link>
  - <Link href="/dev-mode">
  - <UnderlineNavLink px={4} className="d-inline-block">
  + </NextLink>
  + <NextLink href="/dev-mode">
  + <Link px={4} className="d-inline-block text-white">
  Dev mode
  - </UnderlineNavLink>
  </Link>
  - <Link href="/demos">
  - <UnderlineNavLink px={4} className="d-inline-block">
  + </NextLink>
  + <NextLink href="/demos">
  + <Link color="white" px={4} className="d-inline-block text-white">
  Demos
  - </UnderlineNavLink>
  </Link>
  - <Link href="/sandbox">
  - <UnderlineNavLink mr={0} px={4} className="d-inline-block">
  + </NextLink>
  + <NextLink href="/sandbox">
  + <Link color="white" mr={0} px={4} className="d-inline-block text-white">
  Sandbox
  - </UnderlineNavLink>
  </Link>
  + </NextLink>
  </div>
  - }
  - mr={1}
  - >
  - <FlexContainer alignItems="center" ml={5}>
  - <Octicon size="medium" icon={MarkGithub} className="mr-2" />
  - <Text color="black" fontWeight="bold">Primer-react</Text>
  </FlexContainer>
  - </UnderlineNav>
  export default Header
  