Terminal uses a number of icons such as for New Window, Fullscreen, and other actions. These icons should be taken from the user's icon theme if they are available.
Created attachment 2414 Use standards compliant icons if available The built-in icons are still used as fallback if the standard icons are not available, and a warning is shown that the compliant icons aren't there.
Created attachment 2415 Use Gtk stock fullscreen On a related matter, we should use Gtk's fullscreen stock item. This patch doesn't remove the built-in one since it conflicts with the former changes, but I can provide it once the primary patch is done with.
Committed in revision 30114.