I'm confused about what this is asking for?

Tell us what’s happening:
Describe your issue in detail here.
can someone help me with the solution here? I’m not sure what this is asking for

  **Your code so far**
/* file: index.html */
<!DOCTYPE html>
<html>    
<head>
  <title>City Skyline</title>
  <link href="styles.css" rel="stylesheet" type="text/css" />   
</head>

<body>
  <div class="background-buildings">
    <div class="bb1">
      <div class="bb1a"></div>
      <div class="bb1b"></div>
      <div class="bb1c"></div>
      <div class="bb1d"></div>
    </div>
  </div>
</body>
</html>
/* file: styles.css */
* {
border: 1px solid black;
box-sizing: border-box;
}

body {
height: 100vh;
margin: 0;
overflow: hidden;
}

.background-buildings {
width: 100%;
height: 100%;
}

.bb1 {
width: 10%;
height: 70%;
display: flex;
flex-direction: column;
align-items: center;
--building-color1: #999;
}
.bb1a {
width: 70%;
height: 10%;
var(--building-color1): #999;
}
.bb1b {
width: 80%;
height: 10%;
}

.bb1c {
width: 90%;
height: 10%;
}

.bb1d {
width: 100%;
height: 70%;
}
  
  **Your browser information:**

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.5 Safari/605.1.15

Challenge: Step 15

Link to the challenge:

“Add your variable as the value of the background-color property of the .bb1a class.”

You created the variable --building-color1 in the previous step. Now you need to use it for the background-color property in the .bb1a class.