error due to OS.setDarkThemePreferred()

Setting Forbidden Reference to "Error" again while preserving custom
settings.
After investigation, with a complete Target platform, there are no
compile errors and darktheme works. Having consitent settings across all
users ensure we have the same dev environment.

Tested, child eclipse switches to darktheme without issues.

Change-Id: Ib517505e9ac335d839fb212a9d3b4fb42ca420fb
Signed-off-by: Leo Ufimtsev <lufimtse@redhat.com>