tut_35.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS Variables/Custom Properties</title>
    <style>
        :root{
            --box-color:green; 
        }
        .box{
            /* --box-color:green; */
            width: 250px;
            height: 250px;
            background: var(--box-color);
            border: 2px solid red;
            margin: 4px 4px;
        }
        .container{
            display: flex;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
    </div>
</body>
</html>