How do I use the ‘Display Wrapper Attributes’ feature?

The image below is from the ‘Advanced’ tab of a custom composite element in a Drupal Webform. The element is creating an unordered list <ul> in HTML-speak. The list items <li> are, in turn, a list of information. I would like to create some space between the bottom of one <li> item and the top of the next one.

In the ‘Submission Display’ section of the ‘Advanced’ tab of the element I’m building for the webform, is a way to add ‘Display CSS classes’ and/or ‘Display CSS style’. As you can see, I know how to create the ‘custom’ CSS class here; but, then what do I do to tell Drupal or Webform to do stuff WITH the custom class?

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