Learn CSS Variables by Building a City Skyline - Step 10

Tell us what’s happening:
Describe your issue in detail here.

  **Your code so far**
/* file: index.html */
<!DOCTYPE html>
<html lang="en">    
<head>
  <meta charset="UTF-8">
  <title>City Skyline</title>
  <link href="styles.css" rel="stylesheet" />
</head>

<body>
  <div class="background-buildings">
    <div class="bb1"></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%;
}
  
  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/104.0.5112.81 Safari/537.36 Edg/104.0.1293.47

Challenge: Learn CSS Variables by Building a City Skyline - Step 10

Link to the challenge:

Why you don’t have .bb1 in your CSS?

I have tried .bb1. This couldn’t work.

Show me what you’ve tried.