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

Index: Makefile.in
--- Makefile.in.orig
+++ Makefile.in
@@ -396,9 +396,9 @@ Glade_DATA = $(XMLFILE)
 mimedir = $(datadir)/mime
 mimepackagesdir = $(mimedir)/packages
 dist_mimepackages_DATA = grsync.xml
-pixmapdir = $(datadir)/pixmaps
+pixmapdir = $(datadir)/icons/hicolor/128x128/apps/
 pixmap_DATA = pixmaps/grsync.png
-pixmapbusydir = $(datadir)/pixmaps
+pixmapbusydir = $(datadir)/icons/hicolor/128x128/apps/
 pixmapbusy_DATA = pixmaps/grsync-busy.png
 mimeicondir = $(datadir)/icons/hicolor/48x48/mimetypes/
 mimeicon_DATA = pixmaps/application-x-grsync-session.png