Learn CSS Variables by Building a City Skyline - Step 51

What is wrong here?
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></div>
    <div></div>
    <div class="bb1">
      <div class="bb1a bb1-window"></div>
      <div class="bb1b bb1-window"></div>
      <div class="bb1c bb1-window"></div>
      <div class="bb1d"></div>
    </div>
    <div class="bb2">
      <div class="bb2a"></div>
      <div class="bb2b"></div>
    </div>
    <div class="bb3"></div>
    <div></div>
    <div class="bb4"></div>
    <div></div>
    <div></div>
  </div>

  <div class="foreground-buildings">
    <div></div>
    <div></div>
    <div class="fb1"></div>
    <div class="fb2"></div>
    <div></div>
    <div class="fb3"></div>
    <div class="fb4"></div>
    <div class="fb5"></div>
    <div class="fb6"></div>
    <div></div>
    <div></div>
  </div>
</body>
</html>
/* file: styles.css */
:root {
--building-color1: #aa80ff;
--building-color2: #66cc99;
--building-color3: #cc6699;
--building-color4: #538cc6;
--window-color1: black;
--window-color2: #8cd9b3;
}

* {
border: 1px solid black;
box-sizing: border-box;
}

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

.background-buildings, .foreground-buildings {
width: 100%;
height: 100%;
display: flex;
align-items: flex-end;
justify-content: space-evenly;
position: absolute;
top: 0;
}

/* BACKGROUND BUILDINGS - "bb" stands for "background building" */
.bb1 {
width: 10%;
height: 70%;
display: flex;
flex-direction: column;
align-items: center;
}

.bb1a {
width: 70%;
}

.bb1b {
width: 80%;
}

.bb1c {
width: 90%;
}

.bb1d {
width: 100%;
height: 70%;
background: linear-gradient(
    var(--building-color1) 50%,
    var(--window-color1)
  );
}

.bb1-window {
height: 10%;
background: linear-gradient(
    var(--building-color1),
    var(--window-color1)
  );
}

.bb2 {
width: 10%;
height: 50%;
background-color: var(--building-color2);
}
.bb2b {
width: 100%;
height: 100%;
background: linear-gradient(
  var(--building-color2) 0%,
  var(--building-color2) 6%,
  var(--window-color2) 6%
  var(--window-color2) 9%);
}
.bb3 {
width: 10%;
height: 55%;
background-color: var(--building-color3);
}

.bb4 {
width: 11%;
height: 58%;
background-color: var(--building-color4);
}

/* FOREGROUND BUILDINGS - "fb" stands for "foreground building" */
.fb1 {
width: 10%;
height: 60%;
background-color: var(--building-color4);
}

.fb2 {
width: 10%;
height: 40%;
background-color: var(--building-color3);
}

.fb3 {
width: 10%;
height: 35%;
background-color: var(--building-color1);
}

.fb4 {
width: 8%;
height: 45%;
background-color: var(--building-color1);
position: relative;
left: 10%;
}

.fb5 {
width: 10%;
height: 33%;
background-color: var(--building-color2);
position: relative;
right: 10%;
}

.fb6 {
width: 9%;
height: 38%;
background-color: var(--building-color3);
}
  
  **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/104.0.1293.54 LikeWise/97.6.1295.96

Challenge: Learn CSS Variables by Building a City Skyline - Step 51

Link to the challenge:

Hello there.

Do you have a question?

If so, please edit your post to include it in the Tell us what’s happening section.

Learning to describe problems is hard, but it is an important part of learning how to code.

Also, the more information you give us, the more likely we are to be able to help.

getting error message - " You should give .bb2b a background property."
How is there no background property?

  **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></div>
    <div></div>
    <div class="bb1">
      <div class="bb1a bb1-window"></div>
      <div class="bb1b bb1-window"></div>
      <div class="bb1c bb1-window"></div>
      <div class="bb1d"></div>
    </div>
    <div class="bb2">
      <div class="bb2a"></div>
      <div class="bb2b"></div>
    </div>
    <div class="bb3"></div>
    <div></div>
    <div class="bb4"></div>
    <div></div>
    <div></div>
  </div>

  <div class="foreground-buildings">
    <div></div>
    <div></div>
    <div class="fb1"></div>
    <div class="fb2"></div>
    <div></div>
    <div class="fb3"></div>
    <div class="fb4"></div>
    <div class="fb5"></div>
    <div class="fb6"></div>
    <div></div>
    <div></div>
  </div>
</body>
</html>
/* file: styles.css */
:root {
--building-color1: #aa80ff;
--building-color2: #66cc99;
--building-color3: #cc6699;
--building-color4: #538cc6;
--window-color1: black;
--window-color2: #8cd9b3;
}

* {
border: 1px solid black;
box-sizing: border-box;
}

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

.background-buildings, .foreground-buildings {
width: 100%;
height: 100%;
display: flex;
align-items: flex-end;
justify-content: space-evenly;
position: absolute;
top: 0;
}

/* BACKGROUND BUILDINGS - "bb" stands for "background building" */
.bb1 {
width: 10%;
height: 70%;
display: flex;
flex-direction: column;
align-items: center;
}

.bb1a {
width: 70%;
}

.bb1b {
width: 80%;
}

.bb1c {
width: 90%;
}

.bb1d {
width: 100%;
height: 70%;
background: linear-gradient(
    var(--building-color1) 50%,
    var(--window-color1)
  );
}

.bb1-window {
height: 10%;
background: linear-gradient(
    var(--building-color1),
    var(--window-color1)
  );
}

.bb2 {
width: 10%;
height: 50%;
background-color: var(--building-color2);
}
.bb2b {
width: 100%;
height: 100%;
background: linear-gradient(
  var(--building-color2) 0%,
  var(--building-color2) 6%,
  var(--window-color2) 6%,
  var(window-color2) 9%
);
}
.bb3 {
width: 10%;
height: 55%;
background-color: var(--building-color3);
}

.bb4 {
width: 11%;
height: 58%;
background-color: var(--building-color4);
}

/* FOREGROUND BUILDINGS - "fb" stands for "foreground building" */
.fb1 {
width: 10%;
height: 60%;
background-color: var(--building-color4);
}

.fb2 {
width: 10%;
height: 40%;
background-color: var(--building-color3);
}

.fb3 {
width: 10%;
height: 35%;
background-color: var(--building-color1);
}

.fb4 {
width: 8%;
height: 45%;
background-color: var(--building-color1);
position: relative;
left: 10%;
}

.fb5 {
width: 10%;
height: 33%;
background-color: var(--building-color2);
position: relative;
right: 10%;
}

.fb6 {
width: 9%;
height: 38%;
background-color: var(--building-color3);
}
  
  **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/104.0.1293.54 LikeWise/97.6.1295.96

Challenge: Learn CSS Variables by Building a City Skyline - Step 51

Link to the challenge: