<code css>const styles = (theme) => ({
  root: {
    fontFamily: theme.fontFamily,
    width: '100%',
    marginTop: 80,
    marginLeft: 90,
  },
  item: {
    display: 'block',
    float: 'left',
    height: 50,
    backgroundColor: '#e8f5e9',
    padding: '30px 40px 0px 40px',
    margin: '0px 10px 0px 0px',
    textAlign: 'center',
    position: 'relative',
    fontSize: 20,
    color: '#4caf50',
    textDecoration: 'none',
    '&:after': {
      content: '""',
      borderTop: '40px solid red',
      borderBottom: '40px solid red',
      borderLeft: '40px solid blue',
      position: 'absolute',
      right: -40,
      top: 0,
    },
  },
});
</code>

content: '""'

上面这条是重点，如果写成content: ''就不会生效，或者这样写也行content: `{xxx}`