How to create a wdf-custom.css

Hi,
tried to just duplicate and rename a existing style. So I have wdf-note.css and wdf-note-2.css but this caused the dropdown (wp-admin/edit.php?post_type=funder&page=wdf_settings&tab=presentation) to show an empty enty.

What is the trick to add a custom style?

Thanks and cheers, Alex