fdo#71511: in autodetected a11y HC mode, pull background color from theme

- ... and reload ColorConfig after tweaking relevant a11y settings
- dark theme default to high contrast
- as per fdo#35365, having a dark document background is inconvenient
  for non-a11y endusers
  - a11y standard require the (rather ugly) background for a11y
  - thus, when "automatically detect high contract mode of operating
    system"
    in Tools->Options->a11y is enabled, use the dark document
    backgroundby default, otherwise use a white default

note that the autodetect HC option is somewhat broken anyway:
it resets the icon theme hard, so there are not simple roundtrips
enableing/disabling it for that, but those havent been there before
either.

(includes: Ia35a41717224dfb7437054bb885c61d7e0b189d7)

Change-Id: Id8ad1eb3d57b3708ac5a241092208e3a8d98ade0
Reviewed-on: https://gerrit.libreoffice.org/7484
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>