To ensure Jest ignores CSS files when running tests, simply follow the steps below:

  1.  Install a module: `identity-obj-proxy`

npm install -D identity-obj-proxy

2.   Update your jest config(jest.config.json)

{
  "jest":{
     "moduleNameMapper": {
      "\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$": "/__mocks__/fileMock.js",
      "\\.(css|less|scss|sass)$": "identity-obj-proxy"
    },
  }
}

Now run your tests again.   You should not see failures due to CSS files anymore.