EDIT: For close button to be visible on light themes.
<p style="text-align: center;"><a class="btn btn-primary collapsed" data-toggle="collapse" href="#popup">Popup button<br></a></p>
<div class="collapse" id="popup">
<div class="fixed-bottom p-2" style="top:0px;bottom:0px;left:0px;background: rgba(0,0,0,.6);">
<br><br><br><br><br><br><br>
<div class="d-flex justify-content-center">
<div class="col-10 col-md-8 col-lg-6 col-xl-5 col-auto">
<div align="center">
<div class="card mb">
<div class="card-block bg-faded p-3">
<p>Add the popup message's content here.
<br>
<br><a class="btn btn-sm btn-default" data-toggle="collapse" href="#popup">Close Box</a></p>
</div>
</div>
</div>
</div>
</div>
</div>
</div>