fdo#86472 added white backgrounds to images so they appear fine in dark themes

Change-Id: Iaae21bc2205b62520b3dbe6564099e24d87beaf6
Reviewed-on: https://gerrit.libreoffice.org/13142
Reviewed-by: Yousuf Philips <philipz85@hotmail.com>
Tested-by: Yousuf Philips <philipz85@hotmail.com>
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>