Step-7 Reuse style with "css"

App.js

import React, { Component } from 'react';
import styled, { css } from 'styled-components';

...

const border = css`
  ${props => {
    if (props.showBorder) {
      return `
        border: 1px solid ${mainColor};
        border-radius: 8px;
      `;
    }
  }}
`;

const Thumbnail = styled.img`
  flex-grow: 1;
  width: 300px;
  height: 250px;
  padding: 5px;
  margin: 15px;
  ${border}
`;

...