fdo#69046 fix discolored thumbnail and background image of LushGreen template

Change-Id: I71307b9f5d973b8282b793df1d71816fe002b541
Reviewed-on: https://gerrit.libreoffice.org/6667
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>