How to theme webform's Custom Composite element?
You can add classes like you do for any other element. The only difference is that in the case of custom composite elements, there are no fields for setting them directly. Instead, you can go to your respective nested element of Custom composite and set your classes like

'#attributes':
  class: 
    - trip-departure-date


inside the custom property and theme accordingly. You can also use bootstrap class to change the layout.