Learn CSS Variables by Building a City Skyline - Step 15

Tell us what’s happening:
Describe your issue in detail here.
please i need help i am stuck here .

To use a variable, put the variable name in parentheses with var in front of them like this: var(--variable-name). Add your variable as the value of the background-color property of the .bb1a class. Whatever value you gave the variable will be applied to whatever property you use it on. In this case, your variable has the value of #999. So #999 will be used as the value for the background-color property.

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

  <div class="background-buildings">
    <div class="bb1">
      <div class="bb1a"></div>
      <div class="bb1b"></div>
      <div class="bb1c"></div>
      <div class="bb1d"></div>
/* 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%;
background-color: 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 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36

Challenge: Learn CSS Variables by Building a City Skyline - Step 15

Link to the challenge:

it should like this
background-color: var(–building-color1);