...according to alexlarson, for now (until "eventually we'll have a sane dconf
in the sandbox") apps need to specify --filesystem=xdg-run/dconf,
--filesystem=~/.config/dconf:ro, --talk-name=ca.desrt.dconf,
--env=DCONF_USER_CONFIG_DIR=.config/dconf (where the first two are irrelevant
for LO due to its --filesystem=host).

This fixes <https://github.com/flathub/org.libreoffice.LibreOffice/issues/4>
"Libreoffice flatpak is not respecting user theme".

Change-Id: If5ed33e1b524bfa877b89f052c6067398b5600a3