Since gnome-icons-theme in bionic is now provided with lubuntu-icon-theme,...

Since gnome-icons-theme in bionic is now provided with lubuntu-icon-theme, which might require heavy work, my suggestion is to backport cosmic. That is already stripped out from ubuntu's branding.

helpers/make-gnome-icon-theme

+#!/bin/sh
+#
+#    Copyright (C) 2008-2016  Ruben Rodriguez <ruben@trisquel.info>
+#
+#    This program is free software; you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation; either version 2 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program; if not, write to the Free Software
+#    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
+#
+
+VERSION=1
+EXTERNAL='deb-src http://archive.ubuntu.com/ubuntu cosmic universe'
+
+. ./config
+
+changelog "Backport from cosmic"
+
+compile
\ No newline at end of file