background color attribute should only be applied in a dialog
if the user selects it

Change-Id: Idf03840f7332a8cdbbfd616ce47ac26b3602d840
Reviewed-on: https://gerrit.libreoffice.org/20592Reviewed-by: Oliver Specht <oliver.specht@cib.de>
Tested-by: Oliver Specht <oliver.specht@cib.de>