example css injection

<!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>Test</title>
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/css/bootstrap.min.css" integrity="sha384-B0vP5xmATw1+K9KRQjQERJvTumQW0nPEzvF6L/Z6nronJ3oUOFUFpCjEUQouq2+l" crossorigin="anonymous">
</head>

<body>
    <div class="container">
        <div class="row">
            <div class="card card-body col-md-4 offset-md-4 mt-3">
                <h3 class="text-center">Masukan Code HEX Color</h3>
                <div class="form-group mt-2">
                    <input type="text" id="code" class="form-control">
                </div>
                <button class="btn btn-primary" id="sub">Submit</button>
            </div>
        </div>
    </div>

    <script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/dist/js/bootstrap.min.js" integrity="sha384-+YQ4JLhjyBLPDQt//I+STsc9iw4uQqACwlvpslubQzn4u2UU2UFM80nGisd026JF" crossorigin="anonymous"></script>
    <script>
        $(document).ready(function() {
            let code = $('#code');
            code.click(function() {
                $(this).keyup(function() {
                    $(this).attr("value", $(this).val());
                });
            });

            $('#sub').click(function() {
                $('head').append("<style>.card {background-color: " + code.val() + "}</style>");
            });
        });
    </script>
</body>

</html>