Defined additional named colors COL_GRAY3 and COL_gray7 in color.hxx

Fixed the color scheme as required.

Change-Id: Ia86acb2465d0978d08518f0a146df5574cc8fafc
Reviewed-on: https://gerrit.libreoffice.org/21742Reviewed-by: jan iversen <jani@documentfoundation.org>
Tested-by: jan iversen <jani@documentfoundation.org>