Created attachment 7616 Patch for icon change I suggest a more suitable bookmark icon for .desktop files. The new icon has no plus sign. Please have a look at the patch attached.
The patch makes sense from my pov (especially when you compare the two Adwaita icons).
In the past we had the icon "gnome-fs-bookmark" which I think is still the most suitable icon available when I compare the available themes on my Xubuntu 17.10 machine. But this icon has been dropped according to the change log: commit 24a27170604dba7c9e518ce6d0884bb0b5fe819b Author: Sean Davis <smd.seandavis@gmail.com> Date: Sun Jul 2 20:14:49 2017 -0400 Drop non-standard gnome-fs-bookmark and gnome-mime-application-x-executable icons commit 199534217cfc30c342ea6d881ffebf72ad9d45b0 Author: Sean Davis <smd.seandavis@gmail.com> Date: Sun Jul 2 19:47:55 2017 -0400 Drop non-standard gnome-fs-* icons The icon "user-bookmarks" seems to be for a button to list all bookmarks. This icon looks fine with most themes but on Adwaita and Gnome theme we have a directory symbol with a star. The "bookmark-new" icon seems to be for a button to create new bookmarks and not for an existing bookmark. This explains the plus sign.
Manuel Grießmayr referenced this bugreport in commit eb01d3b550d5a8d180e61bdab26029e91beff4d9 Use user-bookmarks instead of bookmark-new for the bookmark icon (bug 14243) https://git.xfce.org/xfce/exo/commit?id=eb01d3b550d5a8d180e61bdab26029e91beff4d9
Thanks for the patch! Applied above.