Resolves fdo#79278: not too dark, not too bright background color

This patch is based on Caolán's one at 68075a61c28a5c72429f78776a822ed45fdcb4a7
I've chosen the background color used in Windows program Paint.NET as
our default background color. It's a nice compromise that doesn't look
out of place anywhere.

Change-Id: I5081201043be4a7ad6152d1ce59451daa4ece1ac
Reviewed-on: https://gerrit.libreoffice.org/10527
Reviewed-by: Stefan Knorr <heinzlesspam@gmail.com>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>