Use css namespace alias

Change-Id: Icb1c96e9652d475438119b481641e0064e26ca16
Reviewed-on: https://gerrit.libreoffice.org/7917
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>