Pseudo class in CSS

Question

I'm currently working on a footer. There are 3 input-elements as you can see in the picture. Now, if you click on a input, the border-top becomes white. And now I want to achieve: if a input-field is filled with text and I click on another element, a green border-top has to appear. Which pseudo class do I have to use?

HTML:

        <div id="footer">
            <ul>
                <li> <input type="text" name="name" placeholder="Name"> </li>
                <li> <input type="text" name="email" placeholder="Email"> </li>
                <li> <input class="last" type="text" name="message" placeholder="Message"> </li>
            </ul>       
        </div>


CSS:

    #footer {
      width: 100%;
      background-color: rgba(0, 0, 0, 0.8);
      text-align: center;
      padding: 20px 0px;
    }

    input {
      padding: 10px 5%;
      background: rgba(0, 0, 0, 0.4);
      outline: none;
      width: 60%;
      margin-bottom: 30px;
      font-family: Roboto-Thin;
      color: white;
      border: none;
      border-bottom: 2px solid rgba(255, 255, 255, 0.0);
      border-top: 2px solid rgba(255, 255, 255, 0.0);
      font-size: 18px;
      transition: 0.2s;
    }

    .last {
      margin-bottom: 0px;
    }

    input:focus {
      border-top: 2px solid white;
    }

  [1]: https://i.sstatic.net/4ZE0K.png

Answer 1

Since you're using placeholders, you can use :not(:placeholder-shown):not(:focus)

#footer {
      width: 100%;
      background-color: rgba(0, 0, 0, 0.8);
      text-align: center;
      padding: 20px 0px;
    }

    input {
      padding: 10px 5%;
      background: rgba(0, 0, 0, 0.4);
      outline: none;
      width: 60%;
      margin-bottom: 30px;
      font-family: Roboto-Thin;
      color: white;
      border: none;
      border-bottom: 2px solid rgba(255, 255, 255, 0.0);
      border-top: 2px solid rgba(255, 255, 255, 0.0);
      font-size: 18px;
      transition: 0.2s;
    }

    .last {
      margin-bottom: 0px;
    }

    input:focus {
      border-top: 2px solid white;
    }

input:not(:placeholder-shown):not(:focus) {
  border-top: 1px solid green;
}

<div id="footer">
  <ul>
    <li> <input type="text" name="name" placeholder="Name"> </li>
    <li> <input type="text" name="email" placeholder="Email"> </li>
    <li> <input class="last" type="text" name="message" placeholder="Message"> </li>
  </ul>
</div>