All code are beautifully written. You can customize easily. Just look the amex/ui-dropdown.html file go to form code with change and use.you would like to edit a specific section of the site, simply find the appropriate label in the and change what you need.If you change the "dropdown" find the following section in the amex/scss/pages/dropdown.scss file
amex bootstrap 4 admin dashboard provide this beautifull with standard dropdown form you can see this documentation