Basic CSS: Use an id Attribute to Style an Element

HTML-CSS
1 
<link href="https://fonts.googleapis.com/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;
  }

  .silver-background {
    background-color: silver;
    
    #cat-photo-form {
      background-color: green;
    }
  }
</style>

<h2 class="red-text">CatPhotoApp</h2>
<main>
  <p class="red-text">Click here to view more <a href="#">cat photos</a>.</p>
  
  <a href="#"><img class="smaller-image thick-green-border" src='//cdck-file-uploads-global.s3.dualstack.us-west-2.amazonaws.com/freecodecamp/original/3X/6/1/61a3499c5abb165be990aa0c1abd23342e60c663.jpg' alt="A cute orange cat lying on its back."></a>
  
  <div class="silver-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" id="cat-photo-form">
    <label><input type="radio" name="indoor-outdoor" checked> Indoor</label>
    <label><input type="radio" name="indoor-outdoor"> Outdoor</label><br>
    <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><br>
    <input type="text" placeholder="cat photo URL" required>
    <button type="submit">Submit</button>
  </form>
</main>

 
I am supposed to change the form background to green, but background color is not changing.
2

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make easier to read.

Note: Backticks are not single quotes.

See this post to find the backtick on your keyboard. The “preformatted text” tool in the editor (</>) will also add backticks around text.

markdown_Forums

3 
  .silver-background {
    background-color: silver;
    
    #cat-photo-form {
      background-color: green;
    }
  }

You did not close the .silver-background { } and you have an extra closing curly braces after #cat-photo-form { }, remove that and place it after .silver-background { }

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