always build an empty DbgUnhandledException, to allow debug=t builds without --enable-dbgutils
fdo#71511: in autodetected a11y HC mode, pull background color from theme

- 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
      background
        by default, otherwise use a white default

Change-Id: Id8ad1eb3d57b3708ac5a241092208e3a8d98ade0