https://github.com/archlinux/svntogit-community/blob/packages/grsync/trunk/use-themed-icon.patch

Index: configure
--- configure.orig
+++ configure
@@ -6020,7 +6020,7 @@ else
 		GTK_API_VERSION="2.0"
 fi
 pkg_modules="gtk+-$GTK_API_VERSION >= 2.16.0"
-OTHER_DESKTOP_ENTRIES="Icon=grsync.png"
+OTHER_DESKTOP_ENTRIES="Icon=grsync"
 MIMEINFO_XMLNS="xmlns='http://www.freedesktop.org/standards/shared-mime-info'"
 MIME_OSSOCAT=""