Created attachment 7322 Add a margin for icons in tasklist There is no margin for icons in tasklist (if we use the Gtk3 Adwaita theme). In my patch I defined a margin of 3px (I don't know, if we need more or less).
Created attachment 7337 Define space between icons and labels in tasklist Instead of using gtk_widget_set_margin_* functions. New patch which defines new CSS property (works fine when panel is horizontal and vertical).
Simon Steinbeiss referenced this bugreport in commit 589515347c797c10374ae1f39d694746707d91b0 tasklist: Define cosmetic default padding for images (Bug #13865) https://git.xfce.org/xfce/xfce4-panel/commit?id=589515347c797c10374ae1f39d694746707d91b0
Thanks Olivier, I should've added this to the panel instead of Greybird from the start. I amended your patch a little (adding a widget name is not necessary, because the css is mapped only to the context of the single widget).