dap-ui-controls icons do not show when :background attribute of fringe face is not set.

[sents]

At the moment the icons of the dap-ui-controls posframe popup do not show up if the
:background property of the fringe face is unset.

dap-ui--create-command constructs the image property for the icons like so

 (image :type png
                               :file ,(f-join dap-ui--control-images-root-dir image)
                               :ascent center
                               :background ,(face-attribute 'fringe :backgound nil t))

Ideally there would be another fallback or documentation telling the user to set the face attribute.