to dodać na górze:
<link href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">

a guzik do powrotu to:

<a  th:href="@{/costam}" >
                        <span>
                            <i class="fas fa-2x fa-chevron-double-left"></i>
                        </span>
                        <span class="mr-md-1">
                            Back
                        </span>

</a>