Babylon.js/Viewer/assets/templates/default/overlay.html

<style>
    .overlay {
        width: 100%;
        height: 100%;
        display: none;
        align-items: center;
        justify-content: center;
        background-color: rgba(121, 121, 121, 0.3);
    }

    error.overlay {
        background-color: rgba(121, 121, 121, 1);
    }

    div#close-button {
        position: absolute;
        top: 10px;
        right: 10px;
        width: 30px;
        height: 30px;
        cursor: pointer;
    }

    div#close-button img {
        width: 100%;
    }
</style>

<div id="close-button">
    <img src="{{closeImage}}" alt="{{closeText}}">
</div>
<help class="overlay"></help>
<error class="overlay"></error>
<share class="overlay"></share>