If the user explicitly gives a background color to the field portion,
then respect that, don't overwrite with the field shading (the UI does
the same).

Change-Id: I7c35618f82a37ef1dd16c03b82651268767813af
Reviewed-on: https://gerrit.libreoffice.org/69127Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins