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="timepicker" width="312" />
<script>
$('#timepicker').timepicker({ uiLibrary: 'materialdesign' });
</script>
Bootstrap 3
<input id="timepicker" width="270" />
<script>
$('#timepicker').timepicker({ uiLibrary: 'bootstrap', modal: false, footer: false });
</script>
Bootstrap 4
<input id="timepicker" width="276" />
<script>
$('#timepicker').timepicker({ uiLibrary: 'bootstrap4' });
</script>
Bootstrap 5
<input id="timepicker" width="276" />
<script>
$('#timepicker').timepicker({ uiLibrary: 'bootstrap5' });
</script>