The name of the UI library that is going to be in use.
The css file for bootstrap should be manually included if you use bootstrap.
Examples
MaterialDesign
<input id="datetimepicker" width="312" />
<script>
$('#datetimepicker').datetimepicker({ uiLibrary: 'materialdesign' });
</script>
MaterialDesign Modal
<input id="datetimepicker" width="312" />
<script>
$('#datetimepicker').datetimepicker({ uiLibrary: 'materialdesign', modal: true, footer: true });
</script>
Bootstrap 3
<input id="datetimepicker" width="220" />
<script>
$('#datetimepicker').datetimepicker({ uiLibrary: 'bootstrap' });
</script>
Bootstrap 4
<input id="datetimepicker" width="234" />
<script>
$('#datetimepicker').datetimepicker({ uiLibrary: 'bootstrap4', modal: true, footer: true });
</script>
Bootstrap 5
<input id="datetimepicker" width="234" />
<script>
$('#datetimepicker').datetimepicker({ uiLibrary: 'bootstrap5', modal: true, footer: true });
</script>