Give a Background Color to a Div Element(元素背景色)

说明：

你可以用 background-color 属性来设置一个元素的背景颜色。

例如，如果你想把一个元素的背景颜色设置为green，你应该把这些加到你的 style 元素中：

.green-background {
  background-color: green;
}
创建一个叫做 gray-background 的类选择器，设置其 background-color 为 gray，最后应用到 div 元素。

原始代码：

<link href="https://fonts.gdgdocs.org/css?family=Lobster" rel="stylesheet" type="text/css">
<style>
  .red-text {
    color: red;
  }

  h2 {
    font-family: Lobster, Monospace;
  }

  p {
    font-size: 16px;
    font-family: Monospace;
  }

  .thick-green-border {
    border-color: green;
    border-width: 10px;
    border-style: solid;
    border-radius: 50%;
  }

  .smaller-image {
    width: 100px;
  }
</style>

<h2 class="red-text">CatPhotoApp</h2>

<p>Click here for <a href="#">cat photos</a>.</p>

<a href="#"><img class="smaller-image thick-green-border" alt="A cute orange cat lying on its back" src="/images/relaxing-cat.jpg"></a>

<div>
  <p>Things cats love:</p>
  <ul>
    <li>cat nip</li>
    <li>laser pointers</li>
    <li>lasagna</li>
  </ul>
  <p>Top 3 things cats hate:</p>
  <ol>
    <li>flea treatment</li>
    <li>thunder</li>
    <li>other cats</li>
  </ol>
</div>

<form action="/submit-cat-photo">
  <label><input type="radio" name="indoor-outdoor" checked> Indoor</label>
  <label><input type="radio" name="indoor-outdoor"> Outdoor</label>
  <label><input type="checkbox" name="personality" checked> Loving</label>
  <label><input type="checkbox" name="personality"> Lazy</label>
  <label><input type="checkbox" name="personality"> Energetic</label>
  <input type="text" placeholder="cat photo URL" required>
  <button type="submit">Submit</button>
</form>

调试代码：

<link href="https://fonts.gdgdocs.org/css?family=Lobster" rel="stylesheet" type="text/css">
<style>
  .red-text {
    color: red;
  }

  h2 {
    font-family: Lobster, Monospace;
  }

  p {
    font-size: 16px;
    font-family: Monospace;
  }

  .thick-green-border {
    border-color: green;
    border-width: 10px;
    border-style: solid;
    border-radius: 50%;
  }

  .smaller-image {
    width: 100px;
  }
  .gray-background{
    background-color:gray;
  }
</style>

<h2 class="red-text">CatPhotoApp</h2>

<p>Click here for <a href="#">cat photos</a>.</p>

<a href="#"><img class="smaller-image thick-green-border" alt="A cute orange cat lying on its back" src="/images/relaxing-cat.jpg"></a>

<div class="gray-background">
  <p>Things cats love:</p>
  <ul>
    <li>cat nip</li>
    <li>laser pointers</li>
    <li>lasagna</li>
  </ul>
  <p>Top 3 things cats hate:</p>
  <ol>
    <li>flea treatment</li>
    <li>thunder</li>
    <li>other cats</li>
  </ol>
</div>

<form action="/submit-cat-photo">
  <label><input type="radio" name="indoor-outdoor" checked> Indoor</label>
  <label><input type="radio" name="indoor-outdoor"> Outdoor</label>
  <label><input type="checkbox" name="personality" checked> Loving</label>
  <label><input type="checkbox" name="personality"> Lazy</label>
  <label><input type="checkbox" name="personality"> Energetic</label>
  <input type="text" placeholder="cat photo URL" required>
  <button type="submit">Submit</button>
</form>