Veja como fazer um login  Modal backdrop login bootstrap 5.1.3 com icone.

Estilo dark.

Código gerado

 <style>
 .big-icon {
 font-size: 10em;
 }
 </style>
 <!-- Modal -->
 <div id="login_modal" class="modal fade" id="staticBackdrop" data-bs-backdrop="static" data-bs-keyboard="false" tabindex="-1" aria-labelledby="staticBackdropLabel" aria-hidden="true">
 <div class="modal-dialog modal-dialog-centered modal-fullscreen-sm-down">
 <div class="modal-content bg-dark bg-gradient opacity-75">

 <div class="modal-body">
 <div class="row">
 <div class="col-12 mx-auto">
 <div class="text-center">
 <i class="bi bi-person-circle text-white big-icon"></i>
 </div>

 <form class="row g-3">
 <div class="col-12">
 <input type="text" class="form-control" name="" value="" placeholder="E-mail">
 </div>
 <div class="col-12">
 <input type="text" class="form-control" name="" value="" placeholder="password">
 </div>
 <div class="col-12">
 <button type="button" name="button" class="btn btn-dark float-end shadow-sm bg-gradient fw-bolder px-5">Enter</button>
 </div>
 </form>

 </div>
 </div>
 </div>
 <div class="modal-footer justify-content-between text-white p-0 px-3">

 <span> <a href="#">Reset password</a></span>
 <span>Not a member yet? <a href="#">Sign Up</a>.</span>
 </div>
 </div>
 </div>
 </div>
 <script>
 window.addEventListener("load", function(event) {
 let myModalElement = document.getElementById('login_modal');
 let myModal = new bootstrap.Modal(myModalElement, {
 keyboard: false
 });
 myModal.show();

 console.log("Todos os recursos terminaram o carregamento!");
 });
 </script>