Let's take the date field as an example.
The function 'wccheckoutfieldsdatepicker_field' is creating the markup for fields of the type 'date'

The classes for the input field are hard coded to 'checkout-date-picker input-text', instead of using '$args['input_class']'.
This way all other classes are discarded.

Using 'woocommerceformfield_args' to add the input classes or a new filter would be really great to be able to alter things.

Right now the addon breaks the styling of some themes.