Form Builder and Form Toolkit are 2 separate tools for 2 separate use cases
Form Builder should be used if you want a tool to create the forms for you.
Form Toolkit should be used if you would prefer to create the forms manually and then add validation to it yourself.
With Form Builder, you can customize the look of the datepicker elements by clicking the Customize CSS button on the initial form builder screen.
In the design editor, Select "Form Elements" from the initial select list, the select "Datepicker" from the secondary select list.
this will open the screen that allows you to customize the look of your datepicker elements in the form.