As agreed in the bug report, a preview shouldn't be
themed by the OS theme.

(cherry picked from commit fa1807ad)

Conflicts:
	include/svx/numvset.hxx
	svx/source/dialog/svxbmpnumvalueset.cxx

Change-Id: Icdc5021c3ac614fe1d490fd513a407a81dbc169e
Reviewed-on: https://gerrit.libreoffice.org/18068Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>
Tested-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>