You can combine these forms with the amazing Bootswatch templating system to create amazing looking forms. Check out this one which uses the Materia design.
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap-icons/font/bootstrap-icons.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootswatch/dist/materia/bootstrap.min.css">
<script src="https://cdn.form.io/js/formio.embed.js?src=https://examples.form.io/example"></script>