The separators have a semi transparent grey color. Also the size of the separator seems to be the same size of the buttons is it separating. Maybe it would be better if the separator have no color/fully transparent and is only 1/4 the width of the buttons/icons. Maybe the separators for action buttons plugin could somehow use the same style as the panel separators for consistency.
Simon Steinbeiss referenced this bugreport in commit a4c980bb8528eba630f607c96e44b43629905d48 action buttons: Fix separator width (Bug #15558) https://gitlab.xfce.org/xfce/xfce4-panel/commit/a4c980bb8528eba630f607c96e44b43629905d48
Thanks for the report, fixed. The separator is a GtkSeparator widget and therefore it follows the theme's standard looks. Other separators in the panel are currently drawn with cairo, maybe this will be revisited later.