The 2.10.0 version from Xfce 4.12 release won't install Xfce-flat theme. themes/flat/Makefile.am contains: SUBDIRS = $(GTK_SUBDIRS) DIST_SUBDIRS = gtk-2.0 themedir = $(datadir)/themes/Xfce-flat I believe SUBDIRS should be set to 'gtk-2.0'.
OK, so 2.10.1 fix it. Thanks!