message: use css prop, create styled Arrow component
diff: import React from 'react'
  import PropTypes from 'prop-types'
  +import styled from 'react-emotion'
  import {Absolute, Box, Button, Caret, PointerBox, Details, Text} from '../../src'
  -const MergeButton = ({numCommits, onClick, primary}) => {
  - const arrowStyles = {
  - content: '',
  - border: '4px solid',
  +const Arrow = styled('span')({
  + display: 'inline-block',
  + verticalAlign: 'middle',
  + border: '5px solid',
  borderRightColor: 'transparent',
  borderLeftColor: 'transparent',
  borderBottomColor: 'transparent',
  - width: '0',
  - height: '0'
  - }
  + width: 0,
  + height: 0
  +})
  +const MergeButton = ({numCommits, onClick, primary}) => {
  const borderStyles = {
  borderTopLeftRadius: '0',
  borderBottomLeftRadius: '0'
  @@ -27,14 +29,14 @@ const MergeButton = ({numCommits, onClick, primary}) => {
  return (
  <div className="BtnGroup">
  - <Button {...buttonSchemeProps} grouped onClick={onClick} style={{borderRight: 0}}>
  + <Button {...buttonSchemeProps} grouped onClick={onClick} css={{borderRight: 0}}>
  Merge Pull Request
  </Button>
  <Details className="details-reset d-flex float-right">
  {({toggle}) => (
  <React.Fragment>
  - <Button is="summary" {...buttonSchemeProps} onClick={toggle} style={borderStyles}>
  - <div className="d-inline-block v-align-middle" style={arrowStyles} />
  + <Button is="summary" {...buttonSchemeProps} onClick={toggle} css={borderStyles}>
  + <Arrow />
  </Button>
  <Absolute mt={1} width={300} zIndex={99999} boxShadow="small">
  <PointerBox caret="top-left">
  