Where am i suppose to type this:--bulding-color1: #999;

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

  **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;
var(--bulding-color1); 
}
.bb1a {
width: 70%;
height: 10%;
}

.bb1b {
width: 80%;
height: 10%;
}

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

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

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.5005.124 Safari/537.36 Edg/102.0.1245.44

Challenge: Step 14

Link to the challenge:

Hi

Your CSS variable is in the right place.

But you have a typo in the variable name:
--bulding-color1