backstop.json

... 
"scenarios": [
    {
      "label": "My Awesome Site",
      "url": "http://localhost:3000/",
      "hideSelectors": [],
      "removeSelectors": [],
      "selectorExpansion": true,
      "selectors": [
        ".App-header",
        ".App-logo"
        ".App-intro"
      ],
      "readyEvent": null,
      "delay": 500,
      "misMatchThreshold" : 0.1,
      "onBeforeScript": "onBefore.js",
      "onReadyScript": "onReady.js"
    }
  ]
...